Enabled support on OpenGL even through the extension is not fully
implemented so that testing with Chromium/Passthrough commmand decoder
is still possible.

BUG=angleproject:1635

Change-Id: Ia417b1779aace1eae19514325701a79cd33f4ef3
Reviewed-on: https://chromium-review.googlesource.com/678479
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>