This allows the implementation object to also check the caps.

BUG=angleproject:930

Change-Id: I5a48ad942eaa5271b60cffd931829296e5d2a767
Reviewed-on: https://chromium-review.googlesource.com/263481Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>