[Chromecast] Fix media session blocking tests.

content/public/browser/media_session.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_
 #define CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_
 
 #include "base/macros.h"
 #include "content/common/content_export.h"
 
 namespace blink {
 namespace mojom {
 enum class MediaSessionAction;
 }  // namespace mojom
 }  // namespace blink
 
 namespace content {
 
+class MediaSessionObserver;
 class WebContents;
 
 // MediaSession manages the media session and audio focus for a given
 // WebContents. There is only one MediaSession per WebContents.
 //
 // MediaSession allows clients to observe its changes via MediaSessionObserver,
 // and allows clients to resume/suspend/stop the managed players.
 class MediaSession {
  public:
   enum class SuspendType {
@@ skipping 29 matching lines @@
   // Let the media session start ducking such that the volume multiplier is
   // reduced.
   virtual void StartDucking() = 0;
 
   // Let the media session stop ducking such that the volume multiplier is
   // recovered.
   virtual void StopDucking() = 0;
 
  protected:
   MediaSession() = default;
+
+ private:
+  friend class MediaSessionObserver;
+
+  virtual void AddObserver(MediaSessionObserver* observer) = 0;
+  virtual void RemoveObserver(MediaSessionObserver* observer) = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_