[gpu] Use SharedImageFormat in MappableBufferTest

As part of BufferFormat to SharedImageFormat conversions, update
MappableBufferTest to use SharedImageFormat. Also update relevant
callsites to pass in SharedImageFormat changing method signatures.

Bug: 445879533
Change-Id: I17dd91f6526d29206a9d4b976d6010f006d6aa66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6980244
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Saifuddin Hitawala <hitawala@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520830}