This patch implements the necessary plumbing to allow WebExternalTextureLayerClient to be used with texture mailboxes, which allows WebGL and Canvas2D to work with ubercompositor.
CC part of this patch requests the client to prepare a mailbox instead of a texture. Compositor bindings handle interaction with Blink as well as callbacks associated with mailboxes being released.
BUG=179371
Review URL: https://chromiumcodereview.appspot.com/
12374028
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@193807 0039d316-1c4b-4281-b951-d872f2087c98