Use //base shmem APIs in media.mojom.VideoBufferHandle.

While it would have been nice to use the existing read_only_shmem_region
union field, it seems non-trivial as there is some capture code that
seems to depend on having a writable shmem region. As such, this CL
only does the simple migration from the Mojo SharedBuffer API to the
//base shmem API.

This also updates the C++ code using the VideoBufferHandle union to
avoid media::mojom::VideoBufferHandle::New(), which has potentially
surprising behavior, in favor of always constructing the union with
the appropriate subtype.

Bug: 1261169
Bug: 1306358
Bug: 1307959
Change-Id: Ifb3e3b0a033d7bda0cd8e1a99e622cc4ceb0e316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3531206
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Tomas Gunnarsson <tommi@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#994084}