mash: Replace ShelfItemDelegate::CanPin with ShelfItem::pinned_by_policy.

ash/common/shelf/shelf_controller.cc

Index: ash/common/shelf/shelf_controller.cc
diff --git a/ash/common/shelf/shelf_controller.cc b/ash/common/shelf/shelf_controller.cc
index a027b98c3d388753d1128b31b125012c7ae8f3aa..78b0874c10fcac3201e2325613338325d6b9b31b 100644
--- a/ash/common/shelf/shelf_controller.cc
+++ b/ash/common/shelf/shelf_controller.cc
@@ -105,11 +105,6 @@ class ShelfItemDelegateMus : public ShelfItemDelegate {
                                        : window_id_to_title_.begin()->second;
   }
 
-  bool CanPin() const override {
-    NOTIMPLEMENTED();
-    return true;
-  }
-
   ShelfMenuModel* CreateApplicationMenu(int event_flags) override {
     return new ShelfMenuModelMus(this);
   }