This requires storing a reference the the Context's Framebuffer map
in the ValidationContext. Likely we'll need to do this as well for
the other non-shared object types.

BUG=angleproject:747

Change-Id: I73ee8b0be3c3b9e54b7e48e49d6f738cf1d926dd
Reviewed-on: https://chromium-review.googlesource.com/407843
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>