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Readback doesn't have all its steps synchronized properly with
impl side painting enabled. This patch prevents the readback
commit from being swapped to screen and avoids using non-readback
commits for readback by adding a SynchronousReadbackState.

This patch also gives forced draws due to checkerboarding its
own ForcedReadback state that behaves independently of readback.

Previously readback and forced draws shared drawing mechanisms,
but we want readbacks to occur ASAP while we want forced draws
to occur inline with the normal frame scheduling.

BUG=276082

Review URL: https://chromiumcodereview.appspot.com/23503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221854 0039d316-1c4b-4281-b951-d872f2087c98