Chrome doesn't allow any client data in its command buffer.  Add an ANGLE
extension to request a context that disallows client data.

BUG=602737

Change-Id: If9d5144daea3c629a73562396000df59a671aad3
Reviewed-on: https://chromium-review.googlesource.com/441986
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>