Refactoring Shader objects

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

BUG=angle:731
Change-Id: I4fc1efeba9df4f873b3d7c8ba6f67f00b6532b9c
Reviewed-on: https://chromium-review.googlesource.com/213803
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Brandon Jones <bajones@chromium.org>