Fix incorrect device pixel ratio used on startup.

Report the correct device pixel ratio to Chromium, when the
QOpenGLWidget gets initialized. The initial value on startup is
incorrect, because Chromium tries to query the screen information,
before a relevant QWindow handle can be returned, and thus defaults to
1.0f.

Change-Id: Ibbedd7b2915840f9716340e582aedbc13406f11a
Task-number: QTBUG-49727
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>