Remove uses of WebGraphicsContext3D from ui/compositor

This removes all uses of blink::WebGraphicsContext3D as the context type
from ui/compositor/ and dependent code, excluding some test harness setup.
The compositor implementation uses gpu::gles2::GLES2Interface*, defined in
gpu/command_buffer/client/, as the context type to avoid dependencies on
the blink repository.

BUG=181120

Review URL: https://codereview.chromium.org/107513008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240826 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240826