Sharing the bindings with the Impl (read-only) allows us to clean out
the applyUniformBuffers method from the Impl class, and make it D3D-
only.

BUG=angleproject:1123

Change-Id: Icb51a90cb227bbbdd83319ea308b68aa1c4c325e
Reviewed-on: https://chromium-review.googlesource.com/293824Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>