Make the UI an observer of downloads.

content/public/browser/android/download_controller_android.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 CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_
 #define CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_
 
 namespace content {
 class DownloadItem;
 class RenderViewHost;
 class WebContents;
 
 // Interface to request GET downloads and send notifications for POST
 // downloads.
 class DownloadControllerAndroid {
  public:
   // Returns the singleton instance of the DownloadControllerAndroid.
   static DownloadControllerAndroid* Get();
 
   // Starts a new download request with Android. Should be called on the
   // UI thread.
   virtual void CreateGETDownload(RenderViewHost* source, int request_id) = 0;
 
   // Should be called when a POST download is started. Notifies the embedding
   // app about the download. Should be called on the UI thread.
-  virtual void OnPostDownloadStarted(WebContents* web_contents,
-                                     DownloadItem* download_item) = 0;
+  virtual void OnPostDownloadStarted(DownloadItem* download_item) = 0;
  protected:
   virtual ~DownloadControllerAndroid() {};
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_