This gives us the choice between D3D9 and D3D11. We can add more
platforms easily using the same patterns.

BUG=angleproject:901

Change-Id: Ia8dc77466bd32f0b8b0d638ae839f41288bfe25b
Reviewed-on: https://chromium-review.googlesource.com/266889Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>