[//cc] Add MappableSI flow to ZeroCopyRasterBufferProvider

This CL adds a flow that uses MappableSharedImage rather than
GpuMemoryBuffer to ZeroCopyRasterBufferProvider. The new codepath is
enabled by default with a Finch killswitch. The changes for the new
flow are the following:

- Rather than creating and mapping a GMB, create and map a MappableSI
- If the MappableSI fails to map, immediately destroy it and zero out
  its mailbox.
- Where the code currently checks for failure by the GMB being null,
  check for failure via the mailbox being zero
- Dump the memory of the mapped region via ScopedMapping rather than the
  GMB

Note: As part of this change, we had to disable ZeroCopyRaster unittests
on Fuchsia due to crbug.com/1485883, a pre-existing issue that this
change tickled due to the change to the tests creating buffers via
(Mappable)SharedImage rather than TestGpuMemoryBufferManager.

Bug: 1431326, 1485883
Change-Id: I7a9875a748a42ca606c8b20d0c671f9531de8e82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4881760
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: vikas soni <vikassoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1200997}

• cc/raster/zero_copy_raster_buffer_provider.cc[diff]
• cc/trees/layer_tree_host_pixeltest_masks.cc[diff]