[//gpu] Have WebGPU fallback check internal usage when detecting write

WebGPUDecoderImpl's fallback representation currently only checks
whether the client's passed-in WebGPU usage includes any write usage
to decide whether to upload the contents of the Dawn texture to the
SharedImage via Skia at the end of the access. However, a client could
pass in read-only usages but writable internal usages, in which case a
write can also occur (e.g., for lazy clearing).

This CL extends this check to check internal usages as well as usages.
It additionally adds a test that fails without the production change:
Doing an access using the fallback adaptor on an uncleared SI with
read-only usages but a writable internal usage should result in the SI
being cleared at the end of the access, such that doing a subsequent
read-only access succeeds rather than resulting in an error as it would
if the SI was uncleared.

Bug: 339171225
Change-Id: Ia3535574162e8f3b86fc36eba409f829fad0310e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5626014
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1315103}