[Chromecast] Fix media session blocking tests.

content/browser/media/session/media_session_impl.h

Index: content/browser/media/session/media_session_impl.h
diff --git a/content/browser/media/session/media_session_impl.h b/content/browser/media/session/media_session_impl.h
index a34699ae2a7bb9f2a8893bb732f3688528da9478..f8112f0f10fff2d7924ae59453639515ffcb1e37 100644
--- a/content/browser/media/session/media_session_impl.h
+++ b/content/browser/media/session/media_session_impl.h
@@ -8,6 +8,7 @@
 #include <stddef.h>
 
 #include <map>
+#include <memory>
 #include <set>
 
 #include "base/callback_list.h"
@@ -85,8 +86,8 @@ class MediaSessionImpl : public MediaSession,
   }
 #endif  // defined(OS_ANDROID)
 
-  void AddObserver(MediaSessionObserver* observer);
-  void RemoveObserver(MediaSessionObserver* observer);
+  void AddObserver(MediaSessionObserver* observer) override;
+  void RemoveObserver(MediaSessionObserver* observer) override;
 
   void NotifyMediaSessionMetadataChange(
       const base::Optional<MediaMetadata>& metadata);