For testing this also converts two unimportant GLenums, gl::BufferUsage
and gl::CullModeFace.

BUG=angleproject:2169

Change-Id: If1e86a97d0fed3fd567303aca6506ec579503076
Reviewed-on: https://chromium-review.googlesource.com/688000
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>