Relanding the language detection.

chrome/browser/extensions/extension_tabs_module.cc

 // Copyright (c) 2009 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/extensions/extension_tabs_module.h"
 
 #include "app/gfx/codec/jpeg_codec.h"
 #include "base/base64.h"
 #include "base/string_util.h"
 #include "chrome/browser/browser.h"
@@ skipping 821 matching lines @@
       return false;
   } else {
     browser = dispatcher()->GetBrowser();
     if (!browser)
       return false;
     contents = browser->tabstrip_model()->GetSelectedTabContents();
     if (!contents)
       return false;
   }
 
-  // Figure out what language |contents| contains. This sends an async call via
-  // the browser to the renderer to determine the language of the tab the
-  // renderer has. The renderer sends back the language of the tab after the
-  // tab loads (it may be delayed) to the browser, which in turn notifies this
-  // object that the language has been received.
-  contents->GetPageLanguage();
+  AddRef();  // Balanced in GotLanguage()
+
+  NavigationEntry* entry = contents->controller().GetActiveEntry();
+  if (entry) {
+    std::string language = entry->language();
+    if (!language.empty()) {
+      // Delay the callback invocation until after the current JS call has
+      // returned.
+      MessageLoop::current()->PostTask(FROM_HERE, NewRunnableMethod(
+          this, &DetectTabLanguageFunction::GotLanguage, language));
+      return true;
+    }
+  }
+  // The tab contents does not know its language yet.  Let's  wait until it
+  // receives it.
   registrar_.Add(this, NotificationType::TAB_LANGUAGE_DETERMINED,
-      NotificationService::AllSources());
-  AddRef();  // balanced in Observe()
+                 Source<RenderViewHost>(contents->render_view_host()));
+
   return true;
 }
 
 void DetectTabLanguageFunction::Observe(NotificationType type,
                                         const NotificationSource& source,
                                         const NotificationDetails& details) {
   DCHECK(type == NotificationType::TAB_LANGUAGE_DETERMINED);
   std::string language(*Details<std::string>(details).ptr());
+  registrar_.Remove(this, NotificationType::TAB_LANGUAGE_DETERMINED, source);
+
+  GotLanguage(language);
+}
+
+void DetectTabLanguageFunction::GotLanguage(const std::string& language) {
   result_.reset(Value::CreateStringValue(language.c_str()));
   SendResponse(true);
-  Release();  // balanced in Run()
+
+  Release();  // Balanced in Run()
 }
 
 // static helpers
 
 static Browser* GetBrowserInProfileWithId(Profile* profile,
                                           const int window_id,
                                           std::string* error_message) {
   for (BrowserList::const_iterator browser = BrowserList::begin();
       browser != BrowserList::end(); ++browser) {
     if ((*browser)->profile() == profile &&
@@ skipping 16 matching lines @@
   if (ExtensionTabUtil::GetTabById(tab_id, profile, browser, tab_strip,
                                    contents, tab_index))
     return true;
 
   if (error_message)
     *error_message = ExtensionErrorUtils::FormatErrorMessage(
         keys::kTabNotFoundError, IntToString(tab_id));
 
   return false;
 }