Remove obsolete Renderbuffer types. 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/201838 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>