Currently we would only check for the color attachment sample count,
which could return incorrect results for depth or stencil-only
Framebuffers.

BUG=angleproject:2108

Change-Id: I378349c91c0139ee507d88fa6a36a86234fea0d4
Reviewed-on: https://chromium-review.googlesource.com/571064Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>