[Cocoa] VideoFullscreenInterfaceContext::m_manager should be a WeakPtr
<rdar://problem/114654278>
Pull request: https://github.com/WebKit/WebKit/pull/17198
Committed 267505@main (4165157020d8): <https://commits.webkit.org/267505@main> Reviewed commits have been landed. Closing PR #17198 and removing active labels.