We can stop exposing this method on the GL-side, now that we only
use it on the D3D back-end.

BUG=angle:781

Change-Id: I3a9ecc277dcd21147881b8780849782f68b3e62e
Reviewed-on: https://chromium-review.googlesource.com/222924Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>