Fix the HighDPI rendering of QQuickWebEngineView

Use the ratio given with the compositor frame instead of taking it
directly from the QWindow to make sure that we scale according to
the rasterizing of the tiles.

Set the scale directly on the DelegatedFrameNode by making it a
QSGTransformNode to avoid creating an extra QSGNode.

Change-Id: I95bd33c7c6d26e0d456d6df188779dc4e82f9d3b
Reviewed-by: Arvid Nilsson <anilsson@blackberry.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>