media: Implement MediaDrmStorageImpl with tests

media/base/android/media_drm_storage.h

 // Copyright 2017 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_ANDROID_MEDIA_DRM_STORAGE_H_
 #define MEDIA_BASE_ANDROID_MEDIA_DRM_STORAGE_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 50 matching lines @@
   virtual void SavePersistentSession(const std::string& session_id,
                                      const SessionData& session_data,
                                      ResultCB result_cb) = 0;
 
   // Loads the persistent session info for |session_id| from the storage.
   virtual void LoadPersistentSession(
       const std::string& session_id,
       LoadPersistentSessionCB load_persistent_session_cb) = 0;
 
   // Removes the persistent session info for |session_id| from the storage.
+  // If the session for |session_id| exists in the storage, it is removed.
+  // Otherwise, this call is a no-op. In both cases, the result will be true.
+  // The result will be false on other unexpected errors, e.g. connection error
+  // to the storage backend.
   virtual void RemovePersistentSession(const std::string& session_id,
                                        ResultCB result_cb) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MediaDrmStorage);
 };
 
 using CreateStorageCB = base::Callback<std::unique_ptr<MediaDrmStorage>()>;
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_ANDROID_MEDIA_DRM_STORAGE_H_