runJavaScript and UserScripts API should be adapted to the new snapshot

RenderFrameHost::ExecuteJavaScript(...) can only be called on chrome:// or chrome-devtools:// URLs, therefore our runJavaScript and UserScripts API are not working.

How to reproduce the bug: try to run tst_qwebenginepage API test.

[1104/083910:FATAL:render_frame_host_impl.cc(314)] Check failed: CanExecuteJavaScript().

https://codereview.chromium.org/1123783002
https://codereview.chromium.org/1258593002

We should use RenderFrameHost::ExecuteJavaScriptInIsolatedWorld(...) or RenderFrameHost::ExecuteJavaScriptForTests(...) instead of the deprecated one.