[EME] Sound continues playing when video's src is changed

[EME] Sound continues playing when video's src is changed
https://bugs.webkit.org/show_bug.cgi?id=158233

Reviewed by Eric Carlson.

When CDMSessionAVFoundation began listening for outputObscuredDueToInsufficientExternalProtection
KVO notifications, it retained the AVPlayer owned by MediaPlayerPrivateAVFoundationObjC, which
caused the AVPlayer to outlive its original owner, and to continue playing even after the
MediaPlayerPrivateAVFoundationObjC had been destroyed.

Rather than observe for outputObscuredDueToInsufficientExternalProtection changes in
CDMSessionAVFoundation, add a backreference from the media player to the session, listen for changes
in the player, and have the player notify the session when the value of that property changes.

* platform/graphics/avfoundation/objc/CDMSessionAVFoundationObjC.h:
(WebCore::CDMSessionAVFoundationObjC::createWeakPtr):
* platform/graphics/avfoundation/objc/CDMSessionAVFoundationObjC.mm:
(WebCore::CDMSessionAVFoundationObjC::CDMSessionAVFoundationObjC):
(-[WebCDMSessionAVFoundationObjCListener initWithParent:player:]): Deleted.
(-[WebCDMSessionAVFoundationObjCListener invalidate]): Deleted.
(-[WebCDMSessionAVFoundationObjCListener observeValueForKeyPath:ofObject:change:context:]): Deleted.
(WebCore::CDMSessionAVFoundationObjC::~CDMSessionAVFoundationObjC): Deleted.
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h:
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::removeSession):
(WebCore::MediaPlayerPrivateAVFoundationObjC::createSession):
(WebCore::MediaPlayerPrivateAVFoundationObjC::outputObscuredDueToInsufficientExternalProtectionChanged):
(WebCore::playerKVOProperties):
(-[WebCoreAVFMovieObserver observeValueForKeyPath:ofObject:change:context:]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@201524 268f45cc-cd09-0410-ab3c-d52691b4dbfc