Make DisplayItemClient an interface

third_party/WebKit/Source/core/paint/PaintLayer.h

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index b49b421e44bd91e9fadcc24eb57a7b3c1f67d9a2..d2d6ad4f006b486d5fea58582070e0347bf71926 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -156,13 +156,13 @@ private:
 // A good example of this is PaintLayerScrollableArea, which can only happen
 // be instanciated for LayoutBoxes. With the current design, it's hard to know
 // that by reading the code.
-class CORE_EXPORT PaintLayer {
+class CORE_EXPORT PaintLayer : public DisplayItemClient {
     WTF_MAKE_NONCOPYABLE(PaintLayer);
 public:
     PaintLayer(LayoutBoxModelObject*, PaintLayerType);
     ~PaintLayer();
 
-    String debugName() const;
+    String debugName() const final;
 
     LayoutBoxModelObject* layoutObject() const { return m_layoutObject; }
     LayoutBox* layoutBox() const { return m_layoutObject && m_layoutObject->isBox() ? toLayoutBox(m_layoutObject) : 0; }
@@ -618,9 +618,6 @@ public:
     ClipRects* previousPaintingClipRects() const { return m_previousPaintingClipRects.get(); }
     void setPreviousPaintingClipRects(ClipRects* clipRects) { m_previousPaintingClipRects = clipRects; }
 
-    // For subsequence display items.
-    DisplayItemClient displayItemClient() const { return toDisplayItemClient(this); }
-
     PaintTiming* paintTiming();
 
 private: