[GTK] Use a RunLoop::Timer to schedule rendering frames in accelerated compositing...

[GTK] Use a RunLoop::Timer to schedule rendering frames in accelerated compositing mode
https://bugs.webkit.org/show_bug.cgi?id=150756

Reviewed by Darin Adler.

Use a RunLoop::Timer instead of a GMainLoopSource for the
accelerated compositing render loop to improve the performance,
since RunLoop::Timer uses a persistent source.
All the logic to schedule rames has been moved to a helper
internal class RenderFrameScheduler.

* WebProcess/WebPage/gtk/LayerTreeHostGtk.cpp:
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::RenderFrameScheduler):
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::~RenderFrameScheduler):
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::start):
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::stop):
(WebKit::shouldSkipNextFrameBecauseOfContinousImmediateFlushes):
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::nextFrame):
(WebKit::LayerTreeHostGtk::RenderFrameScheduler::renderFrame):
(WebKit::LayerTreeHostGtk::LayerTreeHostGtk):
(WebKit::LayerTreeHostGtk::renderFrame):
(WebKit::LayerTreeHostGtk::scheduleLayerFlush):
(WebKit::LayerTreeHostGtk::cancelPendingLayerFlush):
(WebKit::LayerTreeHostGtk::layerFlushTimerFired): Deleted.
* WebProcess/WebPage/gtk/LayerTreeHostGtk.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191854 268f45cc-cd09-0410-ab3c-d52691b4dbfc