[//cc] Streamline GpuImageDecodeCache::TakeOwnershipOfSkImageBacking()

Simplify for RasterContextProvider::GrContext() being always null, which
means that SkImages::BorrowTextureFrom() will return null.

Followup CLs will remove this method by pushing its impl into callsites
and streamlining those callsites as feasible.

Bug: 391648152
Change-Id: I5a6b86959c0db74db66b165032c5e08ef812eb2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965706
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518594}