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This should improve main thread throughput in cases
where we are deferring the BeginMainFrame, but don't
really need to because the swap ack will come back
before the impl thread needs to draw anyway.

BUG=311213

Review URL: https://codereview.chromium.org/222023010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266685 0039d316-1c4b-4281-b951-d872f2087c98