Reland 73532.

chrome/browser/dom_ui/dom_ui.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/dom_ui/dom_ui.h"
 
 #include "base/i18n/rtl.h"
 #include "base/json/json_writer.h"
 #include "base/stl_util-inl.h"
 #include "base/string_number_conversions.h"
@@ skipping 108 matching lines @@
     const std::vector<const Value*>& args) {
   ExecuteJavascript(GetJavascript(function_name, args));
 }
 
 ui::ThemeProvider* DOMUI::GetThemeProvider() const {
   return GetProfile()->GetThemeProvider();
 }
 
 void DOMUI::RegisterMessageCallback(const std::string &message,
                                     MessageCallback *callback) {
-  message_callbacks_.insert(std::make_pair(message, callback));
+  std::pair<MessageCallbackMap::iterator, bool> result =
+      message_callbacks_.insert(std::make_pair(message, callback));
+
+  // Overwrite preexisting message callback mappings.
+  if (register_callback_overwrites() && !result.second)
+    result.first->second = callback;
 }
 
 Profile* DOMUI::GetProfile() const {
   DCHECK(tab_contents());
   return tab_contents()->profile();
 }
 
 RenderViewHost* DOMUI::GetRenderViewHost() const {
   DCHECK(tab_contents());
   return tab_contents()->render_view_host();
@@ skipping 56 matching lines @@
 }
 
 // TODO(viettrungluu): convert to string16 (or UTF-8 std::string?).
 std::wstring DOMMessageHandler::ExtractStringValue(const ListValue* value) {
   string16 string16_value;
   if (value->GetString(0, &string16_value))
     return UTF16ToWideHack(string16_value);
   NOTREACHED();
   return std::wstring();
 }