Refactor ES3 uniform entry points. This should improve validation speed slightly because it allows us to move the ES3-only check into the ES3-only code, and make it a bit simpler. BUG=angleproject:747 BUG=angleproject:1390 Change-Id: I41f9ffef1c6a552fde924e62e481831f07b1503a Reviewed-on: https://chromium-review.googlesource.com/636061 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>