Update the EXT extension to KHR in ANGLE.

BUG=angleproject:2544

Change-Id: Ia647191c3e762d658b0f7e28a34a9ce2cc545f48
Reviewed-on: https://chromium-review.googlesource.com/1054415Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>