Switch Extensions::TabHelper to use WebContents, WebContentsUserData.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index ae81b550e73f3ef0362ce10308bfe59896c84d74..2ea4cd6ab64d13ab867d5a210fcf4ff5e306d2a7 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -122,6 +122,10 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
                                        const FilePath& plugin_path,
                                        bool is_hung) {}
 
+  // Invoked when WebContents::Clone() was used to clone a WebContents.
+  virtual void DidCloneToNewWebContents(WebContents* old_web_contents,
+                                        WebContents* new_web_contents) {}
+
   // Invoked when the WebContents is being destroyed. Gives subclasses a chance
   // to cleanup. At the time this is invoked |web_contents()| returns NULL.
   // It is safe to delete 'this' from here.