Port relevant Chromium code to the Cobalt 25 codebase to allow for
`MediaSource` objects on a Dedicated Worker to render frames on a
`HTMLMediaElement` owned by the main browser thread.

Usage of this feature is gated behind the `MediaSource.EnableInWorkers`
H5VCC flag. In addition, the
`MediaElement.EnableUsingMediaSourceBufferedRange` and
`MediaElement.EnableUsingMediaSourceAttachmentMethods` flags are also
required as pre-requisite features.

This commit does introduce mutex locks to the `SourceBuffer` and `MediaSource`
classes that cannot be fully gated behind H5VCC flags. However, current
usage of both classes is contained to single-threaded usecases. As such,
the mutex lock code should be an effective no-op.

This is based off of the following Chromium commits:

* https://chromium-review.googlesource.com/c/chromium/src/+/2459431
* https://chromium-review.googlesource.com/c/chromium/src/+/2617351
* https://chromium-review.googlesource.com/c/chromium/src/+/2909389

b/338425382