media: Avoid access violation in MojoCdm after connection error

media/base/cdm_session_tracker.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 MEDIA_BASE_CDM_SESSION_TRACKER_H_
 #define MEDIA_BASE_CDM_SESSION_TRACKER_H_
 
 #include <stdint.h>
 
 #include <string>
 #include <unordered_set>
 
 #include "base/logging.h"
 #include "base/macros.h"
 #include "media/base/media_export.h"
 #include "media/base/media_keys.h"
 
 namespace media {
 
 class MEDIA_EXPORT CdmSessionTracker {
  public:
   CdmSessionTracker();
   ~CdmSessionTracker();
 
-  // Add |session_id| to the list of sessions being tracked.
+  // Adds |session_id| to the list of sessions being tracked.
   void AddSession(const std::string& session_id);
 
-  // Remove |session_id| from the list of sessions being tracked.
+  // Removes |session_id| from the list of sessions being tracked.
   void RemoveSession(const std::string& session_id);
 
-  // Call |session_closed_cb| on any remaining sessions in the list and then
+  // Calls |session_closed_cb| on any remaining sessions in the list and then
   // clear the list.
   void CloseRemainingSessions(const SessionClosedCB& session_closed_cb);
 
+  // Returns whether there are any remaining sessions being tracked.
+  bool HasRemainingSessions() const;
+
  private:
   std::unordered_set<std::string> session_ids_;
 
   DISALLOW_COPY_AND_ASSIGN(CdmSessionTracker);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_CDM_SESSION_TRACKER_H_