Move validation of sample counts into the Renderbuffer implementations
because the exact supported sample counts are not always known.

BUG=angleoproject:886

Change-Id: I9c90d9d435e940b852343a29a6aa11d6cb1ad23b
Reviewed-on: https://chromium-review.googlesource.com/255513Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>