This will make it easier to implement fast state switching on Context
change.

BUG=angleproject:2052

Change-Id: I045cc2164200a93215629a2746068e686d7c99ff
Reviewed-on: https://chromium-review.googlesource.com/659234Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>