Dependent state changes happen when the user calls TexImage
on a Texture attached to a Framebuffer. The Framebuffer should be
told 'hey, you should know about this'. Other objects also have
dependent relationships, like VertexArrays and Buffers.

This refactoring uses a binding pointer design, similar to the type
'RefCountObject'. This design fixes the need for manual decoupling
when one or the other is destroyed. The pointers are cleaned up in
destructors, and do no-ops when either the source or dest is missing.

Also move these new classes to a location where they are accessible to
the GL layer; they will be important for framebuffer completeness.

BUG=angleproject:1388

Change-Id: I92610acb85dae6f9c009b8f071e121fde53782ae
Reviewed-on: https://chromium-review.googlesource.com/348953Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>