Bug 987076 - Construct HTMLMediaElement::mTextTrackManager lazily. r=roc

* Remove the TextTrackManager's initialization from the MediaElement's
ctor and add a getter that creates it if we don't have it yet. The public
functions exposed on the MediaElement's IDL now call this getter.
* Also remove the const qualifier from HTMLMediaElement::TextTracks()
as we don't really need it and this function now needs to call
GetOrCreateTextTrackManager() which is non-const.

content/html/content/public/HTMLMediaElement.h

@@ -518,16 +518,14 @@ public:
   AudioChannel MozAudioChannelType() const;
   void SetMozAudioChannelType(AudioChannel aValue, ErrorResult& aRv);
 
-  TextTrackList* TextTracks() const;
+  TextTrackList* TextTracks();
 
   already_AddRefed<TextTrack> AddTextTrack(TextTrackKind aKind,
                                            const nsAString& aLabel,
                                            const nsAString& aLanguage);
 
   void AddTextTrack(TextTrack* aTextTrack) {
-    if (mTextTrackManager) {
-      mTextTrackManager->AddTextTrack(aTextTrack);
-    }
+    GetOrCreateTextTrackManager()->AddTextTrack(aTextTrack);
   }
 
   void RemoveTextTrack(TextTrack* aTextTrack, bool aPendingListOnly = false) {
@@ -871,6 +869,10 @@ protected:
   // and whose text track readiness state is loading.
   void PopulatePendingTextTrackList();
 
+  // Gets a reference to the MediaElement's TextTrackManager. If the
+  // MediaElement doesn't yet have one then it will create it.
+  TextTrackManager* GetOrCreateTextTrackManager();
+
   // The current decoder. Load() has been called on this decoder.
   // At most one of mDecoder and mSrcStream can be non-null.
   nsRefPtr<MediaDecoder> mDecoder;

content/html/content/src/HTMLMediaElement.cpp

@@ -2040,8 +2040,6 @@ HTMLMediaElement::HTMLMediaElement(already_AddRefed<nsINodeInfo>& aNodeInfo)
 
   RegisterFreezableElement();
   NotifyOwnerDocumentActivityChanged();
-
-  mTextTrackManager = new TextTrackManager(this);
 }
 
 HTMLMediaElement::~HTMLMediaElement()
@@ -3963,9 +3961,9 @@ NS_IMETHODIMP HTMLMediaElement::WindowVolumeChanged()
 
 /* readonly attribute TextTrackList textTracks; */
 TextTrackList*
-HTMLMediaElement::TextTracks() const
+HTMLMediaElement::TextTracks()
 {
-  return mTextTrackManager ? mTextTrackManager->TextTracks() : nullptr;
+  return GetOrCreateTextTrackManager()->TextTracks();
 }
 
 already_AddRefed<TextTrack>
@@ -3973,13 +3971,11 @@ HTMLMediaElement::AddTextTrack(TextTrackKind aKind,
                                const nsAString& aLabel,
                                const nsAString& aLanguage)
 {
-  if (mTextTrackManager) {
-    return mTextTrackManager->AddTextTrack(aKind, aLabel, aLanguage,
-                                           TextTrackMode::Hidden,
-                                           TextTrackReadyState::Loaded,
-                                           TextTrackSource::AddTextTrack);
-  }
-  return nullptr;
+  return
+    GetOrCreateTextTrackManager()->AddTextTrack(aKind, aLabel, aLanguage,
+                                                TextTrackMode::Hidden,
+                                                TextTrackReadyState::Loaded,
+                                                TextTrackSource::AddTextTrack);
 }
 
 void
@@ -3990,6 +3986,15 @@ HTMLMediaElement::PopulatePendingTextTrackList()
   }
 }
 
+TextTrackManager*
+HTMLMediaElement::GetOrCreateTextTrackManager()
+{
+  if (!mTextTrackManager) {
+    mTextTrackManager = new TextTrackManager(this);
+  }
+  return mTextTrackManager;
+}
+
 AudioChannel
 HTMLMediaElement::MozAudioChannelType() const
 {