This extension is currently in draft; ANGLE will be the first implementation.

BUG=angleproject:1553
BUG=chromium:630498

Change-Id: I795caa36f722a347cb7924990b60452d51b1e177
Reviewed-on: https://chromium-review.googlesource.com/423435Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>