This change refactors shaders to no longer be dependent on D3D-only concepts.

BUG=angle:731
Change-Id: I1006112f1d31b7e41bd14bd3225ea157b7d6c6c9
Reviewed-on: https://chromium-review.googlesource.com/214467Tested-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>