Unreviewed build fix; The addedFunction and removedFunction arguments to determineChangedTracksFromNewTracksAndOldItems
were reversed, exposing an underlying, existing bug in AudioTrackPrivate and VideoTrack private. Give a default (null)
value to AudioTrackPrivate::m_client and VideoTrackPrivate::m_client and fix the ordering of arguments in
determineChangedTracksFromNewTracksAndOldItems.

* platform/graphics/AudioTrackPrivate.h:
(WebCore::AudioTrackPrivate::AudioTrackPrivate):
* platform/graphics/VideoTrackPrivate.h:
(WebCore::VideoTrackPrivate::VideoTrackPrivate):
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::determineChangedTracksFromNewTracksAndOldItems):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156726 268f45cc-cd09-0410-ab3c-d52691b4dbfc