Split WebGL compatibility into two extensions. The mechanism for requesting extensions is now a separate extension. Added a way to query the extensions that support enabling. BUG=angleproject:1523 Change-Id: I2efaa9f6d67b12ecae325f455404e34ba04d0e7c Reviewed-on: https://chromium-review.googlesource.com/414529 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>