Refactor validation of FramebufferRenderbuffer.

Consolidate a lot of duplicated code into helper methods. Also
move GetTargetTexture to the context class.

Change-Id: I8e3990a5e3c1976cb533941d0a0946466accec30
Reviewed-on: https://chromium-review.googlesource.com/181556
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>