Don't query this as an Impl method, since it exists on the GL level.
Also some related refactorings and cleanups.

BUG=angleproject:1123

Change-Id: I3610bc0db2bcaa96408506e06a65a2f4dab93150
Reviewed-on: https://chromium-review.googlesource.com/293761Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>