It seems conceptually easier to understand that a vk::Framebuffer is
the resource used in graph. Rather than making the GraphResource be
integrated into the FramebufferVk class itself. This means that the
only objects that are graph resources are Vulkan objects: Images,
Buffers, and Framebuffers.

Refactoring change only.

Bug: angleproject:2828
Change-Id: I59d60643182287d4b1fcf9730a3c3a0da5b65973
Reviewed-on: https://chromium-review.googlesource.com/1249561
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>