Make the UI an observer of downloads.

chrome/browser/ui/browser.h

Index: chrome/browser/ui/browser.h
diff --git a/chrome/browser/ui/browser.h b/chrome/browser/ui/browser.h
index 735fd9634929b6fc980d6db2e7073dbbf6ebba71..aadbfcf72c4e8bb3e859b9dd4fbb5289482ccdf5 100644
--- a/chrome/browser/ui/browser.h
+++ b/chrome/browser/ui/browser.h
@@ -486,6 +486,9 @@ class Browser : public TabStripModelObserver,
   void MaybeUpdateBookmarkBarStateForInstantPreview(
       const chrome::search::Mode& mode);
 
+  // Show a download on the download shelf.
+  void ShowDownload(content::DownloadItem* download);

[Randy Smith (Not in Mondays), 2012/12/20 23:09:25]

So an alternate design would have been to have each download shelf observer the
relevant download manager and do the transition appropriate when a download
showed up (i.e. if the download belonged to a tab on that browser, show the
download shelf if not shown, then if the download shelf was shown, show the
download).  Can you say why you went in this direction?  (Not objecting, but
wanting to explore the different alternatives.)

[asanka, 2012/12/20 23:56:19]

a) DownloadShelf objects aren't created for every Browser. They are created
lazily. We'd have to change that so every Browser creates a shelf proactively
just to observe the DownloadManager.

b) There can be multiple Browsers and multiple DownloadShelf objects. Each will
have to decide whether a newly created download belongs on that shelf.

c) Consumers like Android will make it necessary to have a central observer that
notifies the UI anyway.

+
   FullscreenController* fullscreen_controller() {
     return fullscreen_controller_.get();
   }
@@ -584,8 +587,6 @@ class Browser : public TabStripModelObserver,
   virtual void SetFocusToLocationBar(bool select_all) OVERRIDE;
   virtual void RenderWidgetShowing() OVERRIDE;
   virtual int GetExtraRenderViewHeight() const OVERRIDE;
-  virtual void OnStartDownload(content::WebContents* source,
-                               content::DownloadItem* download) OVERRIDE;
   virtual void ViewSourceForTab(content::WebContents* source,
                                 const GURL& page_url) OVERRIDE;
   virtual void ViewSourceForFrame(