This allows Chrome to expose the ETC texture formats in WebGL on top of ANGLE.

BUG=angleproject:1552

Change-Id: I6a6c3912791e121826b2083421e37df7c0dc38e7
Reviewed-on: https://chromium-review.googlesource.com/c/1342420
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>