Create a secondary file thread, FILE_USER_BLOCKING...

content/public/browser/browser_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 CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/callback.h"
@@ skipping 51 matching lines @@
     // This is the thread that interacts with the database.
     DB,
 
     // This is the "main" thread for WebKit within the browser process when
     // NOT in --single-process mode.
     WEBKIT,
 
     // This is the thread that interacts with the file system.
     FILE,
 
+    // Used for file system operations that block user interactions.
+    // Responsiveness of this thread affect users.
+    FILE_USER_BLOCKING,
+
     // Used to launch and terminate Chrome processes.
     PROCESS_LAUNCHER,
 
     // This is the thread to handle slow HTTP cache operations.
     CACHE,
 
     // This is the thread that processes IPC and network messages.
     IO,
 
 #if defined(OS_CHROMEOS)
@@ skipping 180 matching lines @@
  private:
   friend class BrowserThreadImpl;
 
   BrowserThread() {}
   DISALLOW_COPY_AND_ASSIGN(BrowserThread);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_