Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/painted_overlay_scrollbar_layer.cc

 // Copyright 2016 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_overlay_scrollbar_layer.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "cc/base/math_util.h"
@@ skipping 34 matching lines @@
       thumb_length_(scrollbar_->ThumbLength()) {
   DCHECK(scrollbar_->UsesNinePatchThumbResource());
 }
 
 PaintedOverlayScrollbarLayer::~PaintedOverlayScrollbarLayer() {}
 
 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 {
   return scrollbar_->IsOverlay();
 }
 
 bool PaintedOverlayScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
   return true;
 }
 
 ScrollbarOrientation PaintedOverlayScrollbarLayer::orientation() const {
   return scrollbar_->Orientation();
@@ skipping 91 matching lines @@
 
   thumb_resource_ = ScopedUIResource::Create(
       layer_tree_host()->GetUIResourceManager(), UIResourceBitmap(skbitmap));
 
   SetNeedsPushProperties();
 
   return true;
 }
 
 }  // namespace cc