media: Make MediaKeys ref-counted.

media/base/player_tracker.h

 // Copyright 2014 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 MEDIA_BASE_PLAYER_TRACKER_H_
 #define MEDIA_BASE_PLAYER_TRACKER_H_
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
 // An interface for players to register to be notified when a new decryption key
 // becomes available or when the CDM is unset.
 class MEDIA_EXPORT PlayerTracker {
  public:
   virtual ~PlayerTracker();
 
   // Registers player callbacks with the CDM.
   // - |new_key_cb| is fired when a new decryption key becomes available.
-  // - |cdm_unset_cb| is fired when the CDM is detached from the player.
+  // - |cdm_unset_cb| is fired when the CDM is detached from the player. The
+  //   player should stop using the CDM and release any ref-count to the CDM.
   // Returns a registration ID which can be used to unregister a player.
   virtual int RegisterPlayer(const base::Closure& new_key_cb,
                              const base::Closure& cdm_unset_cb) = 0;
 
   // Unregisters a previously registered player. This should be called when
   // the CDM is detached from the player (e.g. setMediaKeys(0)), or when the
   // player is destroyed.
   virtual void UnregisterPlayer(int registration_id) = 0;
 
  protected:
   PlayerTracker();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PlayerTracker);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_PLAYER_TRACKER_H_