Initializes a uniform buffer on ExternalTexture objects that holds data
used by in shaders. Includes modifications to some Mock tests, which
should be using mock external texture objects instead of real ones.
Bug: dawn:1082
Change-Id: I34c02eadaf38aebf22630ac99098e0637ca1279c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/78240
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Brandon1 Jones <brandon1.jones@intel.com>