Update comment on IOThread to point people to the right place if they

chrome/browser/io_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 CHROME_BROWSER_IO_THREAD_H_
 #define CHROME_BROWSER_IO_THREAD_H_
 #pragma once
 
 #include <string>
 
@@ skipping 31 matching lines @@
 class SdchManager;
 class SSLConfigService;
 class TransportSecurityState;
 class URLRequestContext;
 class URLRequestContextGetter;
 class URLSecurityManager;
 }  // namespace net
 
 // Contains state associated with, initialized and cleaned up on, and
 // primarily used on, the IO thread.
+//
+// If you are looking to interact with the IO thread (e.g. post tasks
+// to it or check if it is the current thread), see
+// content::BrowserThread.
 class IOThread : public content::BrowserThreadDelegate {
  public:
   struct Globals {
     Globals();
     ~Globals();
 
     struct MediaGlobals {
       MediaGlobals();
       ~MediaGlobals();
       // MediaInternals singleton used to aggregate media information.
@@ skipping 137 matching lines @@
       system_url_request_context_getter_;
 
   net::SdchManager* sdch_manager_;
 
   base::WeakPtrFactory<IOThread> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOThread);
 };
 
 #endif  // CHROME_BROWSER_IO_THREAD_H_