Make the UI an observer of downloads.

chrome/browser/android/chrome_web_contents_delegate_android.cc

Index: chrome/browser/android/chrome_web_contents_delegate_android.cc
diff --git a/chrome/browser/android/chrome_web_contents_delegate_android.cc b/chrome/browser/android/chrome_web_contents_delegate_android.cc
index 909fd440fc0e85bc9348cdbb0e4d5fd2ded7270d..4ed56212032e30751c5fe35844038d4e8394ed67 100644
--- a/chrome/browser/android/chrome_web_contents_delegate_android.cc
+++ b/chrome/browser/android/chrome_web_contents_delegate_android.cc
@@ -276,16 +276,12 @@ bool ChromeWebContentsDelegateAndroid::CanDownload(
         source, request_id);
     return false;
   }
+  // DownloadControllerAndroid::OnPostDownloadStarted() is called for the
+  // started download by the default DownloadUIController::Delegate
+  // implementation.
   return true;
 }
 
-void ChromeWebContentsDelegateAndroid::OnStartDownload(
-    WebContents* source,
-    content::DownloadItem* download) {
-  content::DownloadControllerAndroid::Get()->OnPostDownloadStarted(
-      source, download);
-}
-
 void ChromeWebContentsDelegateAndroid::DidNavigateToPendingEntry(
     content::WebContents* source) {
   navigation_start_time_ = base::TimeTicks::Now();