Add GL entry points for DiscardBackbufferCHROMIUM

This adds and uses a GL command buffer command for DiscardBackbufferCHROMIUM
instead of having a WebGraphicsContext3D entry point + common/ IPC message.
This is used to deallocate, if possible, the backbuffer when no longer in use.

BUG=181120

Review URL: https://codereview.chromium.org/104823005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240583 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240583