In new back-ends (Vulkan) this will allow us to avoid storing a ref
to the Renderer in the Surface class.

BUG=angleproject:1319

Change-Id: I3b3f50893070d2993e4e91dd82ee539a083b3727
Reviewed-on: https://chromium-review.googlesource.com/419837
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>