[GTK] Timers might never be fired during animations
https://bugs.webkit.org/show_bug.cgi?id=139062

Reviewed by Martin Robinson.

This can happen in old/slow machines where the time to render
layers might take more than 0.016. Since the layer flush timer is
using a higher priority than WebCore timers, when scheduling all
(or several) layer flushes immediately, no other sources with
lower priority are dispatched in the main loop. We could detect if
we are scheduling layer flushes immediately for too long (100ms)
and schedule the next flush after a delay to ensure other sources
with lower priority have a chance to be dispatched. Also use a
lower priority, GDK_PRIORITY_EVENTS is too high, so we use
GDK_PRIORITY_REDRAW - 1 to ensure it's higher than WebCore timers.

* WebProcess/WebPage/gtk/LayerTreeHostGtk.cpp:
(WebKit::LayerTreeHostGtk::LayerTreeHostGtk): Rename
m_lastFlushTime as m_lastImmediateFlushTime.
(WebKit::LayerTreeHostGtk::layerFlushTimerFired): Save the
fireTime before calling flushAndRenderLayers() and compute the
next flush delay based on the elapsed time using monotonically
increasing time instead of current time. Use the target delay
as next flush delay if we have scheduled flushes immediately for
more than 100ms.
(WebKit::LayerTreeHostGtk::flushAndRenderLayers): Do not save the
layer flush time here, it's done in layerFlushTimerFired() so that
we don't need to keep it as a member.
(WebKit::LayerTreeHostGtk::scheduleLayerFlush): Use global
layerFlushTimerPriority.
* WebProcess/WebPage/gtk/LayerTreeHostGtk.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177144 268f45cc-cd09-0410-ab3c-d52691b4dbfc