Make the UI an observer of downloads.

content/browser/android/download_controller_android_impl.h

Index: content/browser/android/download_controller_android_impl.h
diff --git a/content/browser/android/download_controller_android_impl.h b/content/browser/android/download_controller_android_impl.h
index 5096265fe2c9647295e25ad8963897a8957bc36e..7f51d13af2fbc5b992e7c0826ba06addc1f33918 100644
--- a/content/browser/android/download_controller_android_impl.h
+++ b/content/browser/android/download_controller_android_impl.h
@@ -76,8 +76,7 @@ class DownloadControllerAndroidImpl : public DownloadControllerAndroid,
   // DownloadControllerAndroid implementation.
   virtual void CreateGETDownload(RenderViewHost* source,
                                  int request_id) OVERRIDE;
-  virtual void OnPostDownloadStarted(WebContents* web_contents,
-                                     DownloadItem* download_item) OVERRIDE;
+  virtual void OnPostDownloadStarted(DownloadItem* download_item) OVERRIDE;
 
   // DownloadItem::Observer interface.
   virtual void OnDownloadUpdated(DownloadItem* item) OVERRIDE;