Bug 868611 - Implement HTMLTrackElement::UbindFromTree. r=rillian

Update HTMLTrackElement::UnbindFromTree to remove its TextTrack from
its HTMLMediaElement's list of TextTracks.

content/html/content/public/HTMLMediaElement.h

@@ -523,6 +523,10 @@ public:
     mTextTracks->AddTextTrack(aTextTrack);
   }
 
+  void RemoveTextTrack(TextTrack* aTextTrack) {
+    mTextTracks->RemoveTextTrack(*aTextTrack);
+  }
+
 protected:
   class MediaLoadListener;
   class StreamListener;

content/html/content/src/HTMLTrackElement.cpp

@@ -314,9 +314,12 @@ HTMLTrackElement::BindToTree(nsIDocument* aDocument,
 void
 HTMLTrackElement::UnbindFromTree(bool aDeep, bool aNullParent)
 {
-  if (mMediaParent && aNullParent) {
+  if (mMediaParent) {
+    mMediaParent->RemoveTextTrack(mTrack);
+    if (aNullParent) {
       mMediaParent = nullptr;
     }
+  }
 
   nsGenericHTMLElement::UnbindFromTree(aDeep, aNullParent);
 }