Fix broken experimental.viewport.devicePixelRatio

The experimental.viewport.devicePixelRatio should be applied in
addition to any native devicePixelRatio inherent in the window system
or GUI framework.

This patch updates RenderWidgetHostViewQt::OnSwapCompositorFrame to
match the implementation of RenderWidgetHostViewQt::GetScreenInfo and
take the dpiScale (experimental.viewport.devicePixelRatio) into
account.

Change-Id: Ieadd9fbc73d937d5b5a318091c1157b281bd23c9
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>