Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/blink/web_compositor_support_impl.h

Index: cc/blink/web_compositor_support_impl.h
diff --git a/cc/blink/web_compositor_support_impl.h b/cc/blink/web_compositor_support_impl.h
index a303b273122f181ba943af090b5aff455fc24aa8..de88c02287d7fa9035435a2ca9bc8362358db0da 100644
--- a/cc/blink/web_compositor_support_impl.h
+++ b/cc/blink/web_compositor_support_impl.h
@@ -30,16 +30,19 @@ class CC_BLINK_EXPORT WebCompositorSupportImpl
   std::unique_ptr<blink::WebScrollbarLayer> createScrollbarLayer(
       std::unique_ptr<blink::WebScrollbar> scrollbar,
       blink::WebScrollbarThemePainter painter,
-      std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
+      std::unique_ptr<blink::WebScrollbarThemeGeometry>,
+      blink::WebLayer* scroll_layer) override;
   std::unique_ptr<blink::WebScrollbarLayer> createOverlayScrollbarLayer(
       std::unique_ptr<blink::WebScrollbar> scrollbar,
       blink::WebScrollbarThemePainter painter,
-      std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
+      std::unique_ptr<blink::WebScrollbarThemeGeometry>,
+      blink::WebLayer* scroll_layer) override;
   std::unique_ptr<blink::WebScrollbarLayer> createSolidColorScrollbarLayer(
       blink::WebScrollbar::Orientation orientation,
       int thumb_thickness,
       int track_start,
-      bool is_left_side_vertical_scrollbar) override;
+      bool is_left_side_vertical_scrollbar,
+      blink::WebLayer* scroll_layer) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WebCompositorSupportImpl);