Only validate that color and alpha blend funcs are non-constant on D3D renderers. BUG=angleproject:1025 Change-Id: I9c6008cc2f268bda4afa2c67e0b3d3716e77ea31 Reviewed-on: https://chromium-review.googlesource.com/296681 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>