Reland (3rd try): Lazily initialize MessageLoop for faster thread startup

chrome/browser/io_thread.h

 // Copyright (c) 2012 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_IO_THREAD_H_
 #define CHROME_BROWSER_IO_THREAD_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 239 matching lines @@
   // Provide SystemURLRequestContextGetter with access to
   // InitSystemRequestContext().
   friend class SystemURLRequestContextGetter;
 
   friend class test::IOThreadPeer;
 
   // BrowserThreadDelegate implementation, runs on the IO thread.
   // This handles initialization and destruction of state that must
   // live on the IO thread.
   void Init() override;
-  void InitAsync() override;
   void CleanUp() override;
 
   // Initializes |params| based on the settings in |globals|.
   static void InitializeNetworkSessionParamsFromGlobals(
       const Globals& globals,
       net::HttpNetworkSession::Params* params);
 
   void InitializeNetworkOptions(const base::CommandLine& parsed_command_line);
 
   // Sets up TCP FastOpen if enabled via field trials or via the command line.
@@ skipping 209 matching lines @@
   bool is_quic_allowed_by_policy_;
 
   const base::TimeTicks creation_time_;
 
   base::WeakPtrFactory<IOThread> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOThread);
 };
 
 #endif  // CHROME_BROWSER_IO_THREAD_H_