alternate ntp: implement Show/HideBars API to reduce jank when showing/hiding bars

chrome/browser/ui/search/search_model_observer.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_SEARCH_SEARCH_MODEL_OBSERVER_H_
 #define CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_
 
 namespace chrome {
 namespace search {
 
 struct Mode;
 
 // This class defines the observer interface for the |SearchModel|.
 class SearchModelObserver {
  public:
   // Informs the observer that the mode has changed.
   virtual void ModeChanged(const Mode& old_mode, const Mode& new_mode) = 0;
 
+  // Informs the observer that the visibility of top bars (bookmark and info
+  // bars) has changed.
+  virtual void TopBarsVisibilityChanged(const Mode& mode, bool visible) = 0;
+
  protected:
   virtual ~SearchModelObserver() {}
 };
 
 }  // namespace search
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_