cc: Do not post task to notify Scheduler of lost context.

This fixes a race condition where, until the posted task notifying
the Scheduler has been run, the Scheduler will think that there
is an OutputSurface when there really isn't.

I added DCHECKs that fail reliably much closer to the
source in existing unit tests.

BUG=297268

Review URL: https://chromiumcodereview.appspot.com/24040004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224915 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224915