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If the sync point is retired, the scheduler already executed all gl calls before
the sync point, so OnWaitSyncPoint() does not need to do anything. Currently,
OnWaitSyncPoint() always sets the scheduler off and then on. This CL makes
OnWaitSyncPoint() be able to return early.

BUG=332401

Review URL: https://codereview.chromium.org/128123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243932 0039d316-1c4b-4281-b951-d872f2087c98