Use floor time to limit requestAnimationFrame in compositing mode if there are no swaps pending

BUG=114914
TEST=load a page that triggers the compositor and uses rAF but doesn't
cause damage. the callback rate should still be ~60fps


Review URL: http://codereview.chromium.org/9567038

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124731 0039d316-1c4b-4281-b951-d872f2087c98

content/renderer/render_widget.cc

@@ -738,7 +738,11 @@ void RenderWidget::AnimateIfNeeded() {
       base::TimeDelta::FromMilliseconds(16) : base::TimeDelta();
 
   base::Time now = base::Time::Now();
-  if (now >= animation_floor_time_ || is_accelerated_compositing_active_) {
+
+  // animation_floor_time_ is the earliest time that we should animate when
+  // using the dead reckoning software scheduler. If we're using swapbuffers
+  // complete callbacks to rate limit, we can ignore this floor.
+  if (now >= animation_floor_time_ || num_swapbuffers_complete_pending_ > 0) {
     TRACE_EVENT0("renderer", "RenderWidget::AnimateIfNeeded")
     animation_floor_time_ = now + animationInterval;
     // Set a timer to call us back after animationInterval before