[//media] Guard GMBVideoFramePool SCANOUT usage by SI scanout support

GpuMemoryBufferVideoFramePool adds SCANOUT SharedImage usage in contexts
where overlays are not actually supported. As a result, various
SharedImage backings need to support SCANOUT usage even though they
themselves are not actually capable of producing overlays, as they need
to be usable for GpuMemoryBufferVideoFramePool.

This CL guards GpuMemoryBufferVideoFramePool's addition of SCANOUT usage
by the relevant SharedImage support for scanout being present. On all
platforms but Windows, this is simply whether scanout is supported for
SharedImages. On Windows, a specific cap is used as there are cases
where scanout for SharedImages is supported for software VideoFrames in
particular even though it is in general not supported.

For safety, this change is under a killswitch.

Bug: 330865436
Change-Id: If56a96dc8074ea0bfb8debeda63eba4deb2b6b5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5887190
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359912}