[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..f0b10a0a343d62c62acb0df7c521da3212bfaf50 100644
--- a/content/public/browser/media_session.h
+++ b/content/public/browser/media_session.h
@@ -6,6 +6,7 @@
 #define CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_
 
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "content/common/content_export.h"
 
 namespace blink {
@@ -16,6 +17,7 @@ enum class MediaSessionAction;
 
 namespace content {
 
+class MediaSessionObserver;
 class WebContents;
 
 // MediaSession manages the media session and audio focus for a given
@@ -23,7 +25,7 @@ class WebContents;
 //
 // MediaSession allows clients to observe its changes via MediaSessionObserver,
 // and allows clients to resume/suspend/stop the managed players.
-class MediaSession {
+class CONTENT_EXPORT MediaSession {
  public:
   enum class SuspendType {
     // Suspended by the system because a transient sound needs to be played.
@@ -38,7 +40,7 @@ class MediaSession {
   // none is currently available.
   CONTENT_EXPORT static MediaSession* Get(WebContents* contents);
 
-  virtual ~MediaSession() = default;
+  virtual ~MediaSession();
 
   // Resume the media session.
   // |type| represents the origin of the request.
@@ -64,7 +66,17 @@ class MediaSession {
   virtual void StopDucking() = 0;
 
  protected:
-  MediaSession() = default;
+  MediaSession();
+
+  base::ObserverList<MediaSessionObserver>& observers() { return observers_; }
+
+ private:
+  friend class MediaSessionObserver;
+
+  void AddObserver(MediaSessionObserver* observer);
+  void RemoveObserver(MediaSessionObserver* observer);
+
+  base::ObserverList<MediaSessionObserver> observers_;
 };
 
 }  // namespace content