Merge 75078 - Pump the WebCore Timer queue before running animation callbacks

This works around a bug in the Mac UI message pump (72007) that can lead to starvation
of delayed tasks.  To avoid starving WebCore timers indefinitely when using
requestAnimationFrame, this patch explicitly pumps the WebCore Timer queue one each animate() call.

BUG=71735
TEST=open the WebGL aquarium demo (http://webglsamples.googlecode.com/hg/aquarium/aquarium.html) and click on the * next to "Change View". The animation should always happen without multi-second delays.

Review URL: http://codereview.chromium.org/6524030

TBR=jamesr@chromium.org
Review URL: http://codereview.chromium.org/6531009

git-svn-id: svn://svn.chromium.org/chrome/branches/648/src@75187 0039d316-1c4b-4281-b951-d872f2087c98