Make the UI an observer of downloads.

chrome/browser/ui/browser.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_BROWSER_H_
 #define CHROME_BROWSER_UI_BROWSER_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 468 matching lines @@
 
   // Show the first run search engine bubble on the location bar.
   void ShowFirstRunBubble();
 
   // If necessary, update the bookmark bar state according to the instant
   // preview state: when instant preview shows suggestions and bookmark bar is
   // still showing attached, start the animation to hide it.
   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();
   }
 
   extensions::WindowController* extension_window_controller() const {
     return extension_window_controller_.get();
   }
 
  private:
   friend class BrowserTest;
@@ skipping 78 matching lines @@
   virtual void ContentsZoomChange(bool zoom_in) OVERRIDE;
   virtual void WebContentsFocused(content::WebContents* content) OVERRIDE;
   virtual bool TakeFocus(content::WebContents* source, bool reverse) OVERRIDE;
   virtual gfx::Rect GetRootWindowResizerRect() const OVERRIDE;
   virtual void BeforeUnloadFired(content::WebContents* source,
                                  bool proceed,
                                  bool* proceed_to_fire_unload) OVERRIDE;
   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(
       content::WebContents* source,
       const GURL& frame_url,
       const std::string& frame_content_state) OVERRIDE;
   virtual void ShowRepostFormWarningDialog(
       content::WebContents* source) OVERRIDE;
   virtual bool ShouldCreateWebContents(
       content::WebContents* web_contents,
@@ skipping 361 matching lines @@
   bool window_has_shown_;
 
   // Currently open color chooser. Non-NULL after OpenColorChooser is called and
   // before DidEndColorChooser is called.
   scoped_ptr<content::ColorChooser> color_chooser_;
 
   DISALLOW_COPY_AND_ASSIGN(Browser);
 };
 
 #endif  // CHROME_BROWSER_UI_BROWSER_H_