This design follows the similar designs for Program, Framebuffer, etc.
Because of the current design, share a mutable pointer with the Impl
so the patch becomes a bit smaller and easier to review. In a follow-
up patch we can move the shared code into the GL layer.

BUG=angleproject:1159

Change-Id: Ib243e74779f23be51cdca80f1b5c6e5f3e36059d
Reviewed-on: https://chromium-review.googlesource.com/299876Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>