Remove compositor-only entry points from WebGraphicsContext3D

This removes entry points from blink::WebGraphicsContext3D that were only
being used by the compositor.  This interface should have the entry points
needed by WebGL only.

Review URL: https://codereview.chromium.org/134743002

git-svn-id: svn://svn.chromium.org/blink/trunk@165249 bbb929c8-8fbe-4397-9dbb-9b2b20218538