RenderbufferProxySet and FramebufferTextureBindingPointer aren't
necessary any more with our refactored renderbuffer classes and
ownership of attachments by the Framebuffer.

We can also consolidate the FramebufferAttachment and implementation
to a single class, and no longer need to store ref counted objects
in the Framebuffer class directly.

BUG=angle:660

Change-Id: Idcc06dfb42b47242b33494e797a0ba06d6669511
Reviewed-on: https://chromium-review.googlesource.com/201838Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>