Use a direct include of strings headers in ui/, part 2.

ui/base/models/tree_model.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_BASE_MODELS_TREE_MODEL_H_
 #define UI_BASE_MODELS_TREE_MODEL_H_
 
 #include <vector>
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 #include "ui/base/ui_export.h"
 
 namespace gfx {
 class ImageSkia;
 }
 
 namespace ui {
 
 class TreeModel;
 
@@ skipping 71 matching lines @@
   // GetIcons.
   virtual int GetIconIndex(TreeModelNode* node);
 
  protected:
   virtual ~TreeModel() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_MODELS_TREE_MODEL_H_