This moves the uniform query to the back-end. In D3D, this requires
a bit more redesign, especially for matrix uniforms.

Gives about a 10% speed improvement in the GL/NULL uniforms stress
test on Windows (UniformsBenchmark.Run/gl_null_400_vec4).

BUG=angleproject:1390

Change-Id: Idac22a77118e9e94d2f28c585e31ff0bc785ba94
Reviewed-on: https://chromium-review.googlesource.com/623929
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>