[Blink] Have canvas2D transfer to WebGPU require CRPSI

Zero-copy transfer from canvas2D to WebGPU requires that the 2D canvas
have a CanvasResourceProviderSharedImage instance. This requirement is
already implicit in the flow, as the transfer fails if the CRP
returns null from GetBackingClientSharedImageForExternalWrite() (and
CRPSI is the only subclass that returns non-null from that method).
This CL makes this requirement explicit, both for clarity and to enable
trimming the CanvasResourceProvider interface in followup.

Bug: 352263194
Change-Id: Ib7fdf56709723e234fac1d8f179bb59a2ccae755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965151
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518546}