Introduce the PlatformNotificationContext class.

content/browser/notifications/notification_database.h

 // Copyright 2015 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_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_
 #define CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_
 
 #include <stdint.h>
 
-#include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/sequence_checker.h"
 #include "content/common/content_export.h"
 
 class GURL;
 
 namespace leveldb {
 class DB;
 class Env;
 class WriteBatch;
 }
 
 namespace content {
 
 struct NotificationDatabaseData;
 
 // Implementation of the persistent notification database.
 //
-// This class can be constructed on any thread, but method calls must only be
-// made on a thread or sequenced task runner that allows file I/O. The same
-// thread or task runner must be used for all method calls.
+// The database is built on top of a LevelDB database, either in memory or on
+// the filesystem depending on the path passed to the constructor. When writing
+// a new notification, it will be assigned an id guaranteed to be unique for the
+// lifetime of the database.
+//
+// This class must only be used on a thread or sequenced task runner that allows
+// file I/O. The same thread or task runner must be used for all method calls.
 class CONTENT_EXPORT NotificationDatabase {
  public:
   // Result status codes for interations with the database. Will be used for
   // UMA, so the assigned ids must remain stable.
   enum Status {
     STATUS_OK = 0,
 
     // The database, a notification, or a LevelDB key associated with the
     // operation could not be found.
     STATUS_ERROR_NOT_FOUND = 1,
@@ skipping 75 matching lines @@
   State state_;
 
   base::SequenceChecker sequence_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(NotificationDatabase);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_DATABASE_H_