Check that the requested extension is supported by the backend. 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>