Re-land alexbost's experimental Offscreen Tabs API work.

chrome/browser/extensions/extension_tabs_module.h

Index: chrome/browser/extensions/extension_tabs_module.h
diff --git a/chrome/browser/extensions/extension_tabs_module.h b/chrome/browser/extensions/extension_tabs_module.h
index 944de1c7382c2a8f044a405083464ac82cf22337..167caee8e541de352ab4d53c7bd6d540ca1cd593 100644
--- a/chrome/browser/extensions/extension_tabs_module.h
+++ b/chrome/browser/extensions/extension_tabs_module.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -109,8 +109,9 @@ class UpdateTabFunction : public AsyncExtensionFunction,
                           public content::WebContentsObserver {
  public:
   UpdateTabFunction();
- private:
+ protected:
   virtual ~UpdateTabFunction() {}
+ private:
   virtual bool RunImpl() OVERRIDE;
   virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;
   void OnExecuteCodeFinished(int request_id,
@@ -148,7 +149,7 @@ class DetectTabLanguageFunction : public AsyncExtensionFunction,
 };
 class CaptureVisibleTabFunction : public AsyncExtensionFunction,
                                   public content::NotificationObserver {
- private:
+ protected:
   enum ImageFormat {
     FORMAT_JPEG,
     FORMAT_PNG