Remove custom Task implementations in base.

base/threading/thread.h

 // Copyright (c) 2011 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 BASE_THREAD_H_
 #define BASE_THREAD_H_
 #pragma once
 
 #include <string>
 
@@ skipping 164 matching lines @@
   // The thread's message loop.  Valid only while the thread is alive.  Set
   // by the created thread.
   MessageLoop* message_loop_;
 
   // Our thread's ID.
   PlatformThreadId thread_id_;
 
   // The name of the thread.  Used for debugging purposes.
   std::string name_;
 
-  friend class ThreadQuitTask;
+  friend void ThreadQuitHelper();
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREAD_H_