Compositing layer sync should cause deferred repaints to be fired immediately

Compositing layer sync should cause deferred repaints to be fired immediately
https://bugs.webkit.org/show_bug.cgi?id=90401
<rdar://problem/11792028>

Reviewed by Simon Fraser and Antti Koivisto.

If we sync compositing layers and allow the repaint to be deferred, there is time for a
visible flash to occur. Instead, stop the deferred repaint timer and repaint immediately.

No new tests, configuration and timing dependent.

* page/FrameView.cpp:
(WebCore::FrameView::syncCompositingStateForThisFrame):
(WebCore::FrameView::checkStopDelayingDeferredRepaints):
(WebCore::FrameView::stopDelayingDeferredRepaints): Split off from checkStopDelayingDeferredRepaints.
* page/FrameView.h:
(FrameView): Add stopDelayingDeferredRepaints.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121708 268f45cc-cd09-0410-ab3c-d52691b4dbfc