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This can potentially cut the mouse move, mouse wheel, and touch move
latency for platforms that send the UpdateRect_ACK message within the
UpdateRect IPC handler. Today, it seems that windows and mac will
sometimes send the UpdateRect_ACK synchronously, while linux never will.

Patch also includes some extra instrumentation to make this easier to
track that we may or may not want to land.

BUG=none


Review URL: http://codereview.chromium.org/8820001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114141 0039d316-1c4b-4281-b951-d872f2087c98