[Chromecast] Fix media session blocking tests.

content/public/browser/media_session.h

Index: content/public/browser/media_session.h
diff --git a/content/public/browser/media_session.h b/content/public/browser/media_session.h
index 509629f894ad72b8111d6e5a418046463e6d8c5b..8a0fe0aceabe01294bf51f00e41ec46b29807e79 100644
--- a/content/public/browser/media_session.h
+++ b/content/public/browser/media_session.h
@@ -16,6 +16,7 @@ enum class MediaSessionAction;
 
 namespace content {
 
+class MediaSessionObserver;
 class WebContents;
 
 // MediaSession manages the media session and audio focus for a given
@@ -65,6 +66,12 @@ class MediaSession {
 
  protected:
   MediaSession() = default;
+
+ private:
+  friend class MediaSessionObserver;
+
+  virtual void AddObserver(MediaSessionObserver* observer) = 0;
+  virtual void RemoveObserver(MediaSessionObserver* observer) = 0;
 };
 
 }  // namespace content