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 fb9283e30d5568f122ec4f1e7ff115152ffa505e..45aa0400e4a88bbfa2ecae4424a3d7c838e4d1b2 100644
--- a/android_webview/native/aw_web_contents_delegate.cc
+++ b/android_webview/native/aw_web_contents_delegate.cc
@@ -64,11 +64,6 @@ bool AwWebContentsDelegate::CanDownload(content::RenderViewHost* source,
   return false;
 }
 
-void AwWebContentsDelegate::OnStartDownload(WebContents* source,
-                                            content::DownloadItem* download) {
-  NOTREACHED();  // We always return false in CanDownload.
-}
-
 void AwWebContentsDelegate::AddNewContents(content::WebContents* source,
                                            content::WebContents* new_contents,
                                            WindowOpenDisposition disposition,