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..200356b124ea0993e08276306f6a259c592e3d5c 100644
--- a/chrome/browser/ui/search/search_model_observer.h
+++ b/chrome/browser/ui/search/search_model_observer.h
@@ -5,16 +5,17 @@
 #ifndef CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_
 #define CHROME_BROWSER_UI_SEARCH_SEARCH_MODEL_OBSERVER_H_
 
+#include "chrome/browser/ui/search/search_model.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 model's state has changed.
+  virtual void ModelChanged(const SearchModel::State& old_state,
+                            const SearchModel::State& new_state) = 0;
 
  protected:
   virtual ~SearchModelObserver() {}