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>