Part of the refactoring effort to clean up our classes for FBO
attachments and Renderbuffers.

BUG=angle:660

Change-Id: Id23df904f56499568159611b66c8922b9dce6a3d
Reviewed-on: https://chromium-review.googlesource.com/201832Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>