Fix crash on exit of Google Maps

QtWebKit may crash when exiting the new WebGL version of Google maps.

The problem is that under some circumstances the PlatformLayer is destroyed
before the GraphicsLayer that reference it, causing the GraphicsLayer to
access it during destruction.

Change-Id: Ia3d0f337dcb9b8b38e9e23f53bea669bd868f713
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Remove superfluous assert from WebKitQmlPlugin::initializeEngine

The uri parameter is unused in this method and the ones called by it,
so it should not be asserted on.

Task-number: QTBUG-34066
Change-Id: I184fbad3e6e3d4786b4cc4e9f768f1c006309295
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Doc: Update WebView code sample on Qt WebKit landing page

Replace obsolete 'Page' QML type used in the sample code
with 'ScrollView' from Qt Quick Controls.

Task-number: QTBUG-35154
Change-Id: I0a16a6158be028bf8a7975240411841c4e8200ef
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: I21c9fc2f8a0da8410b168f5582849208f188f325