Start separating the text track container from the media controls

Source/core/html/HTMLMediaElement.h

Index: Source/core/html/HTMLMediaElement.h
diff --git a/Source/core/html/HTMLMediaElement.h b/Source/core/html/HTMLMediaElement.h
index f43d5688319a9119da4f3d6f57f8660b90139b7f..e28f1bb2c65b94550d3191f100581295009afab2 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -58,6 +58,7 @@ class MediaController;
 class MediaControls;
 class MediaError;
 class HTMLMediaSource;
+class TextTrackContainer;
 class TextTrackList;
 class TimeRanges;
 class URLRegistry;
@@ -414,6 +415,8 @@ private:
     void ensureMediaControls();
     void configureMediaControls();
 
+    TextTrackContainer& ensureTextTrackContainer();
+
     virtual void* preDispatchEventHandler(Event*) override final;
 
     void changeNetworkStateFromLoadingToIdle();