subtask of bug 264134. By making SourceBufferPrivate::append and SourceBufferPrivate::removeCodedFrames return a NativePromise we can more easily control which thread the SourceBuffer is running on (including a worker thread)
<rdar://problem/118430680>
Pull request: https://github.com/WebKit/WebKit/pull/20517
Committed 270865@main (0a735f7311a8): <https://commits.webkit.org/270865@main> Reviewed commits have been landed. Closing PR #20517 and removing active labels.