Reduce hardcoded D3D11 assumptions in DrawBuffersTest Change-Id: I9b93cc6ee7218a92dda80dbc3f70aec6fef69ded Reviewed-on: https://chromium-review.googlesource.com/276598 Tested-by: Austin Kinross <aukinros@microsoft.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>