GitLab

This removes CompositeAndReadback support from the scheduler, since the
code path is now removed.

R=brianderson
BUG=251936

Review URL: https://codereview.chromium.org/292533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271722 0039d316-1c4b-4281-b951-d872f2087c98