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

chrome/browser/ui/search/search_model_observer.h

Index: chrome/browser/ui/search/search_model_observer.h
diff --git a/chrome/browser/ui/search/search_model_observer.h b/chrome/browser/ui/search/search_model_observer.h
index 970bba80ca9f0d94eb5ec76d9563a95160c211bb..bc57156b93549406acb24896ce7140134a042e98 100644
--- a/chrome/browser/ui/search/search_model_observer.h
+++ b/chrome/browser/ui/search/search_model_observer.h
@@ -16,6 +16,10 @@ class SearchModelObserver {
   // 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() {}
 };