Remove GraphicsLayer::shouldDirectlyCompositeImage().

We have only one compositor implementation, so shouldDirectlyCompositeImage() is
not needed.

Review URL: https://codereview.chromium.org/83123004

git-svn-id: svn://svn.chromium.org/blink/trunk@162680 bbb929c8-8fbe-4397-9dbb-9b2b20218538