Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/painted_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/painted_scrollbar_layer.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "cc/base/math_util.h"
@@ skipping 43 matching lines @@
     AddMainThreadScrollingReasons(
         MainThreadScrollingReason::kScrollbarScrolling);
 }
 
 PaintedScrollbarLayer::~PaintedScrollbarLayer() {}
 
 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();
 }
 
 bool PaintedScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
   return true;
 }
 
 ScrollbarOrientation PaintedScrollbarLayer::orientation() const {
   return scrollbar_->Orientation();
@@ skipping 203 matching lines @@
 
   scrollbar_->PaintPart(&canvas, part, layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();
 
   return UIResourceBitmap(skbitmap);
 }
 
 }  // namespace cc