Re-land: Make GraphicsLayer the DisplayItemClient for scrollbars composited via PaintLayerComposito…

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
index 2a547251ddf0f5e43cdb482ef9b792368685f5d0..0a95658e7ca8eaa22c192de6c9165c7d6087c523 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
@@ -39,6 +39,7 @@ class GraphicsLayer;
 class IntPoint;
 class Page;
 class LayoutPart;
+class Scrollbar;
 class ScrollingCoordinator;
 
 enum CompositingUpdateType {
@@ -212,6 +213,10 @@ private:
 
     void applyOverlayFullscreenVideoAdjustmentIfNeeded();
 
+    // Checks the given graphics layer against the compositor's horizontal and vertical scrollbar
+    // graphics layers, returning the associated Scrollbar instance if any, else nullptr.
+    Scrollbar* graphicsLayerToScrollbar(const GraphicsLayer*) const;
+
     LayoutView& m_layoutView;
     OwnPtr<GraphicsLayer> m_rootContentLayer;