Adds GPUExternalTexture-related types, as well as
Device::ImportExternalTexture. Adds a basic unit and end2end test.
Bug: dawn:728
Change-Id: Iee9533eb872c493a089cccd500748f1a61457407
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/46060
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>