Revert of Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/painted_overlay_scrollbar_layer.cc

Index: cc/layers/painted_overlay_scrollbar_layer.cc
diff --git a/cc/layers/painted_overlay_scrollbar_layer.cc b/cc/layers/painted_overlay_scrollbar_layer.cc
index 4f64e9e3d7dce2176ff0bfe13351fab150df6373..ab8643ecac019f803376b6168a43c2dd7a04778a 100644
--- a/cc/layers/painted_overlay_scrollbar_layer.cc
+++ b/cc/layers/painted_overlay_scrollbar_layer.cc
@@ -50,6 +50,14 @@
 
 int PaintedOverlayScrollbarLayer::ScrollLayerId() const {
   return scroll_layer_id_;
+}
+
+void PaintedOverlayScrollbarLayer::SetScrollLayer(int layer_id) {
+  if (layer_id == scroll_layer_id_)
+    return;
+
+  scroll_layer_id_ = layer_id;
+  SetNeedsFullTreeSync();
 }
 
 bool PaintedOverlayScrollbarLayer::OpacityCanAnimateOnImplThread() const {