Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

third_party/WebKit/Source/core/layout/ScrollAnchor.h

 // Copyright 2015 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 ScrollAnchor_h
 #define ScrollAnchor_h
 
 #include "core/CoreExport.h"
 #include "platform/geometry/DoublePoint.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class LayoutObject;
 class ScrollableArea;
 
 // Scrolls to compensate for layout movements (bit.ly/scroll-anchoring).
 class CORE_EXPORT ScrollAnchor final {
     DISALLOW_NEW();
 
 public:
     ScrollAnchor(ScrollableArea*);
     ~ScrollAnchor();
 
     // The LayoutObject we are currently anchored to. Lazily computed during
     // save() and cached until the next call to clear().
-    const LayoutObject* anchorObject() const { return m_anchorObject; }
+    LayoutObject* anchorObject() const { return m_anchorObject; }
 
     // Invalidates the anchor.
-    void clear() { m_anchorObject = nullptr; }
+    void clear();
 
     // Records the anchor's location in relation to the scroller. Should be
     // called when the scroller is about to be laid out.
     void save();
 
     // Scrolls to compensate for any change in the anchor's relative location
     // since the most recent call to save(). Should be called immediately after
     // the scroller has been laid out.
     void restore();
 
     DEFINE_INLINE_TRACE() { visitor->trace(m_scroller); }
 
 private:
     // The scroller that owns and is adjusted by this ScrollAnchor.
     RawPtrWillBeMember<ScrollableArea> m_scroller;
 
     // The LayoutObject we should anchor to. Lazily computed.
-    const LayoutObject* m_anchorObject;
+    LayoutObject* m_anchorObject;
 
     // Location of m_layoutObject relative to scroller at time of save().
     DoublePoint m_savedRelativeOffset;
 };
 
 } // namespace blink
 
 #endif // ScrollAnchor_h