Previously, we would only check that the extension was requestable and
not verify that the driver could support it.

BUG=angleproject:1523

Change-Id: I5273920fc08a20beda54d4afaeb406c6c1c2056f
Reviewed-on: https://chromium-review.googlesource.com/723559
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>