Initial implementation of AudioTrack, AudioTrackList, VideoTrack, and VideoTrackList.

Source/core/html/track/TextTrackList.idl

Index: Source/core/html/track/TextTrackList.idl
diff --git a/Source/core/html/track/TextTrackList.idl b/Source/core/html/track/TextTrackList.idl
index f9d982d77dbd7b561d1a30ac989e38753e501f57..02d3577d62df94639e2ae0c6045fa7e3314f49cd 100644
--- a/Source/core/html/track/TextTrackList.idl
+++ b/Source/core/html/track/TextTrackList.idl
@@ -24,15 +24,14 @@
  */
 
 [
-    SetWrapperReferenceFrom=owner,
     RuntimeEnabled=VideoTrack,
+    SetWrapperReferenceFrom=owner,
 ] interface TextTrackList : EventTarget {
     readonly attribute unsigned long length;
     getter TextTrack item(unsigned long index);
-    TextTrack getTrackById(DOMString id);
+    TextTrack? getTrackById(DOMString id);
 
     attribute EventHandler onaddtrack;
     attribute EventHandler onchange;
     attribute EventHandler onremovetrack;
-};
-
+};