This is a refactoring patch only, and doesn't change the behaviour.
The intent is to make it easy to turn on the Framebuffer dirty bits
in a subsequent patch, once we can cleanly handle textures and
renderbuffers getting recreated.

BUG=angleproject:1260

Change-Id: Iaa5cfe222b020724e088eee5f1ae909b6f981a08
Reviewed-on: https://chromium-review.googlesource.com/325423Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>