Implement minimal WebGPU functions to test fences

This patch gets some fence tests working to check that commands are
properly sent between the WebGPUImplementation and WebGPUDecoder.

Bug: 877147
Change-Id: I4387ba4d802b3312d4570c346744685e54a0baf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1481618
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639821}
11 files changed