Move code that applies to audio and video tracks into TrackBase.

Source/core/html/track/TextTrackList.cpp

Index: Source/core/html/track/TextTrackList.cpp
diff --git a/Source/core/html/track/TextTrackList.cpp b/Source/core/html/track/TextTrackList.cpp
index 0d00ba33fee5220d99c603292f4bc7a5808b24e2..ba8ca1aea8f759f1a53575c5f100be8f5627e0d7 100644
--- a/Source/core/html/track/TextTrackList.cpp
+++ b/Source/core/html/track/TextTrackList.cpp
@@ -192,8 +192,8 @@ void TextTrackList::append(PassRefPtr<TextTrack> prpTrack)
 
     invalidateTrackIndexesAfterTrack(track.get());
 
-    ASSERT(!track->mediaElement() || track->mediaElement() == m_owner);
-    track->setMediaElement(m_owner);
+    ASSERT(!track->trackList());
+    track->setTrackList(this);
 
     scheduleAddTrackEvent(track.release());
 }
@@ -220,8 +220,8 @@ void TextTrackList::remove(TextTrack* track)
 
     invalidateTrackIndexesAfterTrack(track);
 
-    ASSERT(track->mediaElement() == m_owner);
-    track->setMediaElement(0);
+    ASSERT(track->trackList() == this);
+    track->setTrackList(0);
 
     tracks->remove(index);