Revert of mash: Remove ChromeLauncherController's |id_to_item_controller_map_|.

ash/common/shelf/shelf_model_observer.h

Index: ash/common/shelf/shelf_model_observer.h
diff --git a/ash/common/shelf/shelf_model_observer.h b/ash/common/shelf/shelf_model_observer.h
index 9953513c9c8cbb63c821fc01e7194fb355457195..194e202675ce111c1b70522a39f44110ff8528fe 100644
--- a/ash/common/shelf/shelf_model_observer.h
+++ b/ash/common/shelf/shelf_model_observer.h
@@ -11,6 +11,7 @@
 namespace ash {
 
 struct ShelfItem;
+class ShelfItemDelegate;
 
 class ASH_EXPORT ShelfModelObserver {
  public:
@@ -25,8 +26,16 @@
   // of the arguments.
   virtual void ShelfItemMoved(int start_index, int target_index) = 0;
 
-  // Invoked after an item changes. |old_item| is the item before the change.
+  // Invoked when the state of an item changes. |old_item| is the item
+  // before the change.
   virtual void ShelfItemChanged(int index, const ShelfItem& old_item) = 0;
+
+  // Gets called when a ShelfItemDelegate gets changed. Note that
+  // |item_delegate| can be null.
+  // NOTE: This is added a temporary fix for M39 to fix crbug.com/429870.
+  // TODO(skuhne): Find the real reason for this problem and remove this fix.
+  virtual void OnSetShelfItemDelegate(ShelfID id,
+                                      ShelfItemDelegate* item_delegate) = 0;
 
  protected:
   virtual ~ShelfModelObserver() {}