Refactor checks for incomplete framebuffer attachments. BUG=722684 Change-Id: I6f47d92b450a056a2344767bef28e96c4552f98e Reviewed-on: https://chromium-review.googlesource.com/506927 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>