Enables MediaSource, SourceBuffer, and SourceBufferList objects to be created
on Dedicated Workers. Creation of these objects is not gated behind any H5VCC
flags, but they will be non-transferable without usage of H5VCC flags.

The initial implementation of CrossThreadMediaSourceAttachment is very bare
bones, and does not allow for MediaElements to interact with the attached
MediaSources yet. However, this lays the groundwork for supporting creating
the relevant MediaSource objects on the worker thread, and the transfer
process to get the reference back to the main thread.

Usage of creating MediaSource URLs is gated through the
`MediaSource.EnableInWorkers` H5VCC flag, in addition to existing flags
`MediaElement.EnableUsingMediaSourceBufferedRange` and
`MediaElement.EnableUsingMediaSourceAttachmentMethods`. If and only if all
three flags are enabled, MediaSources created on Dedicated Workers and passed
to `URL.createObjectURL` will return a non-empty string that can be sent to
the main thread, and used to 'access' the MediaSourceAttachment. In addition,
if and only if all three H5VCC flags are enabled,
[`MediaSource.canConstructInDedicatedWorker`](https://developer.mozilla.org/en-US/docs/Web/API/MediaSource/canConstructInDedicatedWorker_static)
will return true.

This is based on the following Chromium commit:

• https://chromium-review.googlesource.com/c/chromium/src/+/2407075

b/338425449