Add some extensions that are required by the passthrough command decoder
and exposed on all other ANGLE backends.

BUG=781164

Change-Id: Ie83143effd2460122e7a093c658f217fb460d8b2
Reviewed-on: https://chromium-review.googlesource.com/759132Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>