[Mac] Playback target clients do not unregister on page reload

[Mac] Playback target clients do not unregister on page reload
https://bugs.webkit.org/show_bug.cgi?id=144761

Reviewed by Brady Eidson.

* dom/Document.cpp:
(WebCore::Document::prepareForDestruction): Unregister all target picker clients.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::registerWithDocument): Register for page cache callback.
(WebCore::HTMLMediaElement::unregisterWithDocument): Unregister for page cache callback.
(WebCore::HTMLMediaElement::documentWillSuspendForPageCache): New.
(WebCore::HTMLMediaElement::documentDidResumeFromPageCache): New.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184001 268f45cc-cd09-0410-ab3c-d52691b4dbfc