Make HistoryDBTask not refcounted, and ensure it's destroyed on its origin thread.

chrome/browser/history/history_db_task.h

 // Copyright 2013 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 CHROME_BROWSER_HISTORY_HISTORY_DB_TASK_H_
 #define CHROME_BROWSER_HISTORY_HISTORY_DB_TASK_H_
 
-#include "base/memory/ref_counted.h"
-
 namespace history {
 
 class HistoryBackend;
 class HistoryDatabase;
 
 // HistoryDBTask can be used to process arbitrary work on the history backend
 // thread. HistoryDBTask is scheduled using HistoryService::ScheduleDBTask.
 // When HistoryBackend processes the task it invokes RunOnDBThread. Once the
 // task completes and has not been canceled, DoneRunOnMainThread is invoked back
-// on the main thread.
-class HistoryDBTask : public base::RefCountedThreadSafe<HistoryDBTask> {
+// on the main thread, after which this object is destroyed, also on the
+// main thread.
+class HistoryDBTask {
  public:
+  virtual ~HistoryDBTask() {}
+
   // Invoked on the database thread. The return value indicates whether the
   // task is done. A return value of true signals the task is done and
   // RunOnDBThread should NOT be invoked again. A return value of false
   // indicates the task is not done, and should be run again after other
   // tasks are given a chance to be processed.
   virtual bool RunOnDBThread(HistoryBackend* backend, HistoryDatabase* db) = 0;
 
   // Invoked on the main thread once RunOnDBThread has returned true. This is
   // only invoked if the request was not canceled and returned true from
   // RunOnDBThread.
   virtual void DoneRunOnMainThread() = 0;
-
- protected:
-  friend class base::RefCountedThreadSafe<HistoryDBTask>;
-
-  virtual ~HistoryDBTask() {}
 };
 
 }  // namespace history
 
 #endif  // CHROME_BROWSER_HISTORY_HISTORY_DB_TASK_H_