Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/solid_color_scrollbar_layer.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/solid_color_scrollbar_layer.h"
 
 #include <memory>
 
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/solid_color_scrollbar_layer_impl.h"
@@ skipping 81 matching lines @@
 }
 
 bool SolidColorScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
   return true;
 }
 
 int SolidColorScrollbarLayer::ScrollLayerId() const {
   return solid_color_scrollbar_layer_inputs_.scroll_layer_id;
 }
 
-void SolidColorScrollbarLayer::SetScrollLayer(int layer_id) {
-  if (layer_id == solid_color_scrollbar_layer_inputs_.scroll_layer_id)
-    return;
-
-  solid_color_scrollbar_layer_inputs_.scroll_layer_id = layer_id;
-  SetNeedsFullTreeSync();
-}
-
 ScrollbarOrientation SolidColorScrollbarLayer::orientation() const {
   return solid_color_scrollbar_layer_inputs_.orientation;
 }
 
 }  // namespace cc