Make the UI an observer of downloads.

chrome/browser/extensions/extension_host.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 147 matching lines @@
   virtual void RunFileChooser(
       content::WebContents* tab,
       const content::FileChooserParams& params) OVERRIDE;
   virtual void AddNewContents(content::WebContents* source,
                               content::WebContents* new_contents,
                               WindowOpenDisposition disposition,
                               const gfx::Rect& initial_pos,
                               bool user_gesture,
                               bool* was_blocked) OVERRIDE;
   virtual void CloseContents(content::WebContents* contents) OVERRIDE;
-  virtual void OnStartDownload(content::WebContents* source,
-                               content::DownloadItem* download) OVERRIDE;
   virtual void RequestMediaAccessPermission(
       content::WebContents* web_contents,
       const content::MediaStreamRequest& request,
       const content::MediaResponseCallback& callback) OVERRIDE;
 
   // content::NotificationObserver
   virtual void Observe(int type,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
@@ skipping 76 matching lines @@
 
   // Used to measure how long it's been since the host was created.
   PerfTimer since_created_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHost);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_H_