Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index 7fc6e9c1ae8997e9efadb082de31efb0321491e4..75cd6db29e6269eb57ad2dd33f078271509cd83c 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -61,14 +61,6 @@ int PaintedScrollbarLayer::ScrollLayerId() const {
   return scroll_layer_id_;
 }
 
-void PaintedScrollbarLayer::SetScrollLayer(int layer_id) {
-  if (layer_id == scroll_layer_id_)
-    return;
-
-  scroll_layer_id_ = layer_id;
-  SetNeedsFullTreeSync();
-}
-
 bool PaintedScrollbarLayer::OpacityCanAnimateOnImplThread() const {
   return scrollbar_->IsOverlay();
 }