Draw main frame custom scrollbars in root layer scrolling mode.

third_party/WebKit/Source/core/frame/RootFrameViewport.h

Index: third_party/WebKit/Source/core/frame/RootFrameViewport.h
diff --git a/third_party/WebKit/Source/core/frame/RootFrameViewport.h b/third_party/WebKit/Source/core/frame/RootFrameViewport.h
index bebbadfe155ad76cc2c61c7787ad9daa19a89fba..b2f2e5e0f57b66275b0ca233a4d0acf8254f0187 100644
--- a/third_party/WebKit/Source/core/frame/RootFrameViewport.h
+++ b/third_party/WebKit/Source/core/frame/RootFrameViewport.h
@@ -10,8 +10,10 @@
 
 namespace blink {
 
+class CompositedLayerMapping;
 class FrameView;
 class LayoutRect;
+class LocalFrame;
 
 // ScrollableArea for the root frame's viewport. This class ties together the
 // concepts of layout and visual viewports, used in pinch-to-zoom. This class
@@ -77,6 +79,7 @@ public:
     GraphicsLayer* layerForScrolling() const override;
     GraphicsLayer* layerForHorizontalScrollbar() const override;
     GraphicsLayer* layerForVerticalScrollbar() const override;
+    GraphicsLayer* layerForScrollCorner() const override;
     ScrollResult userScroll(ScrollGranularity, const FloatSize&) override;
     bool scrollAnimatorEnabled() const override;
     HostWindow* getHostWindow() const override;
@@ -100,6 +103,9 @@ private:
 
     void distributeScrollBetweenViewports(const DoublePoint&, ScrollType, ScrollBehavior, ViewportToScrollFirst);
 
+    LocalFrame* mainFrame() const;
+    CompositedLayerMapping* compositedLayerMapping() const;
+
     // If either of the layout or visual viewports are scrolled explicitly (i.e. not
     // through this class), their updated offset will not be reflected in this class'
     // animator so use this method to pull updated values when necessary.