The upstream WebGPU spec decided to not pursue CreateBufferMappedAsync,
and it adds some complexity to Dawn, so we remove it.
Bug: dawn:22
Change-Id: I4182a90c4a1aa0bfbaecd7d8f67d7049cf5df5d6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/17321
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>