Explicitly make BrowserThread::SetDelegate specific to the IO thread.

content/public/browser/browser_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 CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 204 matching lines @@
 
   // If the current message loop is one of the known threads, returns true and
   // sets identifier to its ID.  Otherwise returns false.
   static bool GetCurrentThreadIdentifier(ID* identifier) WARN_UNUSED_RESULT;
 
   // Callers can hold on to a refcounted task runner beyond the lifetime
   // of the thread.
   static scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunnerForThread(
       ID identifier);
 
-  // Sets the delegate for the specified BrowserThread.
+  // Sets the delegate for BrowserThread::IO.
   //
-  // Only one delegate may be registered at a time.  Delegates may be
+  // This only supports the IO thread as it doesn't work for potentially
+  // redirected threads (ref. http://crbug.com/653916) and also doesn't make
+  // sense for the UI thread.
+  //
+  // Only one delegate may be registered at a time. The delegate may be
   // unregistered by providing a nullptr pointer.
   //
-  // If the caller unregisters a delegate before CleanUp has been
-  // called, it must perform its own locking to ensure the delegate is
-  // not deleted while unregistering.
-  static void SetDelegate(ID identifier, BrowserThreadDelegate* delegate);
+  // If the caller unregisters the delegate before CleanUp has been called, it
+  // must perform its own locking to ensure the delegate is not deleted while
+  // unregistering.
+  static void SetIOThreadDelegate(BrowserThreadDelegate* delegate);
 
   // Use these templates in conjunction with RefCountedThreadSafe or scoped_ptr
   // when you want to ensure that an object is deleted on a specific thread.
   // This is needed when an object can hop between threads
   // (i.e. IO -> FILE -> IO), and thread switching delays can mean that the
   // final IO tasks executes before the FILE task's stack unwinds.
   // This would lead to the object destructing on the FILE thread, which often
   // is not what you want (i.e. to unregister from NotificationService, to
   // notify other objects on the creating thread etc).
   template<ID thread>
@@ skipping 44 matching lines @@
  private:
   friend class BrowserThreadImpl;
 
   BrowserThread() {}
   DISALLOW_COPY_AND_ASSIGN(BrowserThread);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_