aura: Implement new app list mock.

ash/app_list/app_list_view_delegate.h

Index: ash/app_list/app_list_view_delegate.h
diff --git a/ash/app_list/app_list_view_delegate.h b/ash/app_list/app_list_view_delegate.h
index 875fa6b92171262277ab88603dabc65504fb6281..33039abeb7392392d6bba96468f002f3320d30a2 100644
--- a/ash/app_list/app_list_view_delegate.h
+++ b/ash/app_list/app_list_view_delegate.h
@@ -6,17 +6,24 @@
 #define ASH_APP_LIST_APP_LIST_VIEW_DELEGATE_H_
 #pragma once
 
+#include <string>
+
 #include "ash/ash_export.h"
 
 namespace ash {
 
 class AppListItemModel;
+class AppListModel;
 
 class ASH_EXPORT AppListViewDelegate {
  public:
   // AppListView owns the delegate.
   virtual ~AppListViewDelegate() {}
 
+  // Invoked to ask the delegate to populate the |model| for given |query|.
+  virtual void BuildAppListModel(const std::string& query,
+                                 AppListModel* model) = 0;
+
   // Invoked an AppListeItemModelView is  activated by click or enter key.
   virtual void OnAppListItemActivated(AppListItemModel* item,
                                       int event_flags) = 0;