cc: Poll for state that triggers draws (synchronous compositor only)
This adds Scheduler::PollForAnticipatedDrawTriggers which is used
by the synchronous compositor to avoid requesting BeginImplFrames
when we might not actually draw new content, but still need to
monitor state that might trigger a draw at vsync intervals.
This can occur, for example, when the last swap used incomplete tiles
(checkerboards or low-quality) or when a commit is aborted and we
want to wait a vsync before starting the next commit.
BUG=284810
Review URL: https://chromiumcodereview.appspot.com/
23463014
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222711 0039d316-1c4b-4281-b951-d872f2087c98