PlaybackManager's ref counting was only used to keep it alive in a few
callbacks. Instead, the callbacks can use weak references that can only
be used from the thread that the PlaybackManager was created on, to
ensure that the PlaybackManager can't be destroyed while being
accessed.
This ensures that:
- The PlaybackManager is destroyed immediately when it is reassigned
by HTMLMediaElement
- No callbacks are invoked after that point
This fixes the crash initially being addressed by #8081. The test from
that PR has been included as a regression test.
