cc: setting a needs_commit_ to true is only needed for forced commit when there is no scheduled BeginMainFrame()

Setting a needs_commit_ to true is not needed when there is already scheduled
BeginMainFrame() because scheduled BeginMainFrame() should be excuted after
finishing current requested readback draw.

R=brianderson@chromium.org
BUG=347370
TEST=cc_unittests --gtest_filter=SchedulerStateMachineTest.DontMakeNewCommitAfterDrawingReplaceCommit

Review URL: https://codereview.chromium.org/179553006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254085 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254085