ash: Add GetItemIndexForType() function to ShelfModel.

ash/shelf/shelf_model_util.cc

-// Copyright 2013 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "ash/shelf/shelf_model_util.h"
-
-#include "ash/shelf/shelf_model.h"
-
-namespace ash {
-
-int GetShelfItemIndexForType(LauncherItemType type,
-                             const ShelfModel& shelf_model) {
-  for (size_t i = 0; i < shelf_model.items().size(); i++) {
-    if (shelf_model.items()[i].type == type)
-      return i;
-  }
-  return -1;
-}
-
-}  // namespace ash