QtWebEngine crashes when receiving a QEvent::TouchCancel

The crash is easily reproducible programmatically by calling QWindowSystemInterface::handleTouchCancelEvent(); on a QtQuick window with a WebView and with an active touch.

The crash was introduced by QTBUG-59999, specifically with change https://codereview.qt-project.org/#/q/Ia492e58b83d3ad38cdf6877d468724c399f34367,n,z in RenderWidgetHostViewQt::handleTouchEvent():

TouchCancel events have an empty touchPoints() list, which trips when accessing touchPoints[0]. More generally, that code is missing handling for the TouchCancel case altogether. It should be forwarded to m_touchSelectionController->WillHandleTouchEvent(motionEvent) as well, and respecting Chromium's expectation of the cancel event containing the last event's touchpoints, just like it's done a few lines below.