Avoid use of a raw pointer in WKMediaSessionCoordinatorForTesting.
<rdar://problem/117388768>
Pull request: https://github.com/WebKit/WebKit/pull/19472
Committed 269877@main (d2db744604a4): <https://commits.webkit.org/269877@main> Reviewed commits have been landed. Closing PR #19472 and removing active labels.