Have content/ create and destroy its own threads.

content/browser/browser_thread_impl.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 CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 #define CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 #pragma once
 
+#include "base/synchronization/lock.h"
+#include "base/threading/thread.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/browser_thread.h"
 
 namespace content {
 
-class CONTENT_EXPORT BrowserThreadImpl : public BrowserThread {
+class CONTENT_EXPORT BrowserThreadImpl
+    : public BrowserThread, public base::Thread {
  public:
+  // Construct a BrowserThreadImpl with the supplied identifier.  It is an error
+  // to construct a BrowserThreadImpl that already exists.
   explicit BrowserThreadImpl(BrowserThread::ID identifier);
+
+  // Special constructor for the main (UI) thread and unittests. We use a dummy
+  // thread here since the main thread already exists.
   BrowserThreadImpl(BrowserThread::ID identifier, MessageLoop* message_loop);
   virtual ~BrowserThreadImpl();
 
  private:
-  // We implement most functionality on the public set of
-  // BrowserThread functions, but state is stored in the
-  // BrowserThreadImpl to keep the public API cleaner. Therefore make
-  // BrowserThread a friend class.
+  // We implement all the functionality of the public BrowserThread
+  // functions, but state is stored in the BrowserThreadImpl to keep
+  // the API cleaner. Therefore make BrowserThread a friend class.
   friend class BrowserThread;
 
   // TODO(brettw) remove this variant when Task->Closure migration is complete.
   static bool PostTaskHelper(
       BrowserThread::ID identifier,
       const tracked_objects::Location& from_here,
       Task* task,
       int64 delay_ms,
       bool nestable);
   static bool PostTaskHelper(
       BrowserThread::ID identifier,
       const tracked_objects::Location& from_here,
       const base::Closure& task,
       int64 delay_ms,
       bool nestable);
+
+  // Common initialization code for the constructors.
+  void Initialize();
+
+  // The identifier of this thread.  Only one thread can exist with a given
+  // identifier at a given time.
+  ID identifier_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_