[//chromeos] Have ARC video encode use MappableSI under killswitch

This CL changes the Arc video encode accelerator to create VideoFrames
backed by mappable SharedImages rather than directly by
GpuMemoryBufferHandles. For safety we put this change under a
killswitch. The killswitch is disabled in this CL; a followup CL will
enable it.

Bug: 417658667
Change-Id: I68f7f4e701648b5dbbd5e8a54a2ce6ec5e491c35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6765170
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1489421}