Allow WebGpu Swapbuffers ReleaseCallback to be destroyed on any thread

The callback can be held in ExternalCanvasResource provider that can be
used on main thread and worker thread in case of offscreen canvas with
placeholder. While the callback will never run on the wrong thread, we
can't guarantee that it won't be destroyed on a wrong thread, because
the worker thread that owns it could be destroyed by javascript.

Bug: 352263194
Change-Id: I8af66aea9ed1e70dd7e832772fdd4a1bce6ed70f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6513314
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457132}