Make the UI an observer of downloads.

chrome/browser/download/download_item_model.h

Index: chrome/browser/download/download_item_model.h
diff --git a/chrome/browser/download/download_item_model.h b/chrome/browser/download/download_item_model.h
index 880622682ef5d57e53b81eceac6b2a1c91ace362..8b0f79da45698d4324f5a71b73bf64b9dbcd375f 100644
--- a/chrome/browser/download/download_item_model.h
+++ b/chrome/browser/download/download_item_model.h
@@ -90,6 +90,16 @@ class DownloadItemModel {
   // displayed in the shelf.
   void SetShouldShowInShelf(bool should_show);
 
+  // Returns |true| if the UI should be notified when the download is ready to
+  // be presented in the UI. By default, this value is |false| and should be
+  // changed explicitly using SetShouldNotifyUI(). Note that this is indpendent
+  // of ShouldShowInShelf() since there might be actions other than showing in
+  // the shelf that the UI must perform.
+  bool ShouldNotifyUI() const;
+
+  // Change what's returned by ShouldNotifyUI().
+  void SetShouldNotifyUI(bool should_notify);
+
   content::DownloadItem* download() { return download_; }
 
  private: