[REG Qt 5.6 -> Qt 5.7] QtWebEngineProcessd crashes when pressing mouse's thumb button

1. Run a debug build of the Quick Nano browser example (qtwebengine\examples\webengine\quicknanobrowser\quicknanobrowser.pro)
   It starts showing http://www.qt.io/
2. Click on a link.
3. Click the mouse's rear thumb button which is assigned to the "Back" action.
   While the browser keeps running, QtWebEngineProcessd crashes:

   1   blink::MouseEvent::create                                                                                                                                                                                                       Qt5WebEngineCored  0x14273da1 
   2   blink::EventHandler::updatePointerTargetAndDispatchEvents                                                                                                                                                                       Qt5WebEngineCored  0x1421db8e 
   3   blink::EventHandler::handleMousePressEvent                                                                                                                                                                                      Qt5WebEngineCored  0x1421326e 
   4   blink::PageWidgetEventHandler::handleMouseDown                                                                                                                                                                                  Qt5WebEngineCored  0x11e7df80 
   5   blink::WebViewImpl::handleMouseDown                                                                                                                                                                                             Qt5WebEngineCored  0x11e07f52 
   6   blink::PageWidgetDelegate::handleInputEvent                                                                                                                                                                                     Qt5WebEngineCored  0x11e7ddba 
   7   blink::WebViewImpl::handleInputEvent                                                                                                                                                                                            Qt5WebEngineCored  0x11e0736a 
   8   content::RenderWidgetInputHandler::HandleInputEvent                                                                                                                                                                             Qt5WebEngineCored  0x119999ee 
   9   content::RenderWidget::OnHandleInputEvent                                                                                                                                                                                       Qt5WebEngineCored  0x118ad873 
   10  base::DispatchToMethodImpl<content::RenderWidget,void (__thiscall content::RenderWidget:: *)(blink::WebInputEvent const *,ui::LatencyInfo const &),blink::WebInputEvent const *,ui::LatencyInfo,0,1>                            Qt5WebEngineCored  0x118a427d 
   11  base::DispatchToMethod<content::RenderWidget,void (__thiscall content::RenderWidget:: *)(blink::WebInputEvent const *,ui::LatencyInfo const &),blink::WebInputEvent const *,ui::LatencyInfo>                                    Qt5WebEngineCored  0x118a3e68 
   12  InputMsg_HandleInputEvent::Dispatch<content::RenderWidget,content::RenderWidget,void,void (__thiscall content::RenderWidget:: *)(blink::WebInputEvent const *,ui::LatencyInfo const &)>                                         Qt5WebEngineCored  0x118a39c4 
   13  content::RenderWidget::OnMessageReceived                                                                                                                                                                                        Qt5WebEngineCored  0x118addf2 
   14  content::RenderViewImpl::OnMessageReceived                                                                                                                                                                                      Qt5WebEngineCored  0x1185b871 
   15  content::MessageRouter::RouteMessage                                                                                                                                                                                            Qt5WebEngineCored  0x123b6f2b 
   16  content::ChildThreadImpl::ChildThreadMessageRouter::RouteMessage                                                                                                                                                                Qt5WebEngineCored  0x138ccd53 
   17  content::MessageRouter::OnMessageReceived                                                                                                                                                                                       Qt5WebEngineCored  0x123b6e8d 
   18  content::ChildThreadImpl::OnMessageReceived                                                                                                                                                                                     Qt5WebEngineCored  0x138cc7ed 
   19  base::internal::RunnableAdapter<bool (__thiscall content::ChildThreadImpl:: *)(IPC::Message const &)>::Run                                                                                                                      Qt5WebEngineCored  0x118d8701 
   20  base::internal::InvokeHelper<0,void,base::internal::RunnableAdapter<bool (__thiscall content::ChildThreadImpl:: *)(IPC::Message const &)>,base::internal::TypeList<content::RenderThreadImpl *,IPC::Message const &>>::MakeItSo Qt5WebEngineCored  0x118d6376 
   ... <Mehr>                                                                                                            

QtWebEngineProcessd should not crash. A release build of the same example does not seem to cause a crash in this situation.