Clean up BoxClipper

Source/core/paint/BoxClipper.h

 // Copyright 2014 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 BoxClipper_h
 #define BoxClipper_h
 
 #include "platform/geometry/LayoutPoint.h"
 #include "platform/graphics/paint/DisplayItem.h"
 
 namespace blink {
 
 class LayoutBox;
 struct PaintInfo;
 
 enum ContentsClipBehavior { ForceContentsClip, SkipContentsClipIfPossible };
 
 class BoxClipper {
 public:
-    BoxClipper(LayoutBox&, const PaintInfo&, const LayoutPoint& accumulatedOffset, ContentsClipBehavior);
+    BoxClipper(const LayoutBox&, const PaintInfo&, const LayoutPoint& accumulatedOffset, ContentsClipBehavior);
     ~BoxClipper();
-
-    bool pushedClip() { return m_pushedClip; }
 private:
-    bool m_pushedClip;
-    LayoutPoint m_accumulatedOffset;
+    const LayoutBox& m_box;
     const PaintInfo& m_paintInfo;
-    LayoutBox& m_box;
     DisplayItem::Type m_clipType;
 };
 
 } // namespace blink
 
 #endif // BoxClipper_h