This simplifies uniform management in D3D11. It will also facilitate
further optimizations.

Improves performance in a uniforms stress test by ~13% on a test
machine. (UniformsBenchmark.Run/d3d11_null_400_vec4)

BUG=angleproject:1390

Change-Id: Iba2c15d420396aa8fb4e8c451cba2b4dde7b4b77
Reviewed-on: https://chromium-review.googlesource.com/623930Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>