RealtimeMediaSource does not need to be CanMakeWeakPtr
<rdar://problem/105071795>
Pull request: https://github.com/WebKit/WebKit/pull/9670
Committed 259938@main (f461cca4322d): <https://commits.webkit.org/259938@main> Reviewed commits have been landed. Closing PR #9670 and removing active labels.
Re-opened since this is blocked by bug 252084
Reverted in https://commits.webkit.org/260138@main because this caused the flaky assertion failures documented in https://bugs.webkit.org/show_bug.cgi?id=252029