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This CL adds the groundwork for video overlays on fuchsia:
- Adds feature flags for an alternative path for displaying video
as overlays through an ImagePipe on fuchsia.
- Makes use of these flags for the initial step of associating video
BufferCollections with an ImagePipe before their allocation. There is
one-to-map mapping between BufferCollections and ImagePipe.
ImagePipe is owned by SysmemBufferCollection instance.

Bug: 1127984
Change-Id: Iaa8fa7961b7f1601d7af6a19eaf23f0f73eb7aab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415381Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812120}