Make the UI an observer of downloads.

android_webview/native/aw_web_contents_delegate.cc

Index: android_webview/native/aw_web_contents_delegate.cc
diff --git a/android_webview/native/aw_web_contents_delegate.cc b/android_webview/native/aw_web_contents_delegate.cc
index e08a65f6702caf8a5d28939038ce311b81f136a9..ef9355034abcd2f74fbec4d6421cc440a28e0209 100644
--- a/android_webview/native/aw_web_contents_delegate.cc
+++ b/android_webview/native/aw_web_contents_delegate.cc
@@ -61,11 +61,6 @@ bool AwWebContentsDelegate::CanDownload(content::RenderViewHost* source,
   return false;
 }
 
-void AwWebContentsDelegate::OnStartDownload(WebContents* source,
-                                            content::DownloadItem* download) {
-  NOTREACHED();  // Downloads are cancelled in ResourceDispatcherHostDelegate.
-}
-
 void AwWebContentsDelegate::AddNewContents(content::WebContents* source,
                                            content::WebContents* new_contents,
                                            WindowOpenDisposition disposition,