[//gpu] Add DawnImageRep::BeginScopedAccess(..., internal_usage)

This CL adds a variant of DawnImageRepresentation::BeginScopedAccess()
that takes in internal usage to be passed to the Dawn texture that will
be created. We add only the variant that will actually be needed, as
this callsite [1] is the only one that will need to pass in non-zero
internal usages.

This is in support of changing Dawn representations' current hardcoding
of internal usages to come from the client. The chain will look as
follows, with a fair amount of parallelism in the steps:

* Add support for AssociateMailbox() taking in internal usages
* Modify callers of AssociateMailbox() to pass appropriate internal
  usages
* Have WebGPUDecoderImpl pass internal usages from client through to
  Dawn rep via BeginScopedAccess()
* Pass internal usages down to individual Dawn reps via BeginAccess()
* Under a killswitch, change individual Dawn reps to use
  client-supplied internal usages instead of their current
  hardcoded internal usages

[1] https://source.chromium.org/chromium/chromium/src/+/main:gpu/command_buffer/service/webgpu_decoder_impl.cc;l=1975-1977?q=webgpu_decoder_impl.cc&ss=chromium

Bug: 339171225
Change-Id: I9d1e112f248a90ba111d17245f687234a1d4967e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5589104
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1308710}