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

cc/layers/scrollbar_layer_unittest.cc

 // Copyright 2012 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 <stddef.h>
 
 #include <unordered_map>
 
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 290 matching lines @@
   root_layer->SetBounds(gfx::Size(100, 50));
   content_layer->SetBounds(gfx::Size(100, 50));
 
   layer_tree_host_->SetRootLayer(root_clip_layer);
   root_clip_layer->AddChild(root_layer);
   root_layer->AddChild(content_layer);
   root_layer->AddChild(scrollbar_layer);
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(70, 10));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_location(gfx::Point(20, 10));
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(30, 10, 50, 10));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
   scrollbar_layer->fake_scrollbar()->set_thumb_length(4);
   LayerImpl* root_clip_layer_impl = nullptr;
   PaintedScrollbarLayerImpl* scrollbar_layer_impl = nullptr;
 
   UPDATE_AND_EXTRACT_LAYER_POINTERS();
   EXPECT_EQ(gfx::Rect(10, 0, 4, 10).ToString(),
             scrollbar_layer_impl->ComputeThumbQuadRect().ToString());
@@ skipping 18 matching lines @@
   root_layer->SetBounds(gfx::Size(100, 50));
   content_layer->SetBounds(gfx::Size(100, 50));
 
   layer_tree_host_->SetRootLayer(root_clip_layer);
   root_clip_layer->AddChild(root_layer);
   root_layer->AddChild(content_layer);
   root_layer->AddChild(scrollbar_layer);
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(70, 10));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_location(gfx::Point(20, 10));
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(30, 10, 50, 10));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
   scrollbar_layer->fake_scrollbar()->set_thumb_length(4);
   layer_tree_host_->UpdateLayers();
   LayerImpl* root_clip_layer_impl = nullptr;
   PaintedScrollbarLayerImpl* scrollbar_layer_impl = nullptr;
 
   // Thumb is at the edge of the scrollbar (should be inset to
   // the start of the track within the scrollbar layer's
@@ skipping 53 matching lines @@
   root_layer->SetScrollClipLayerId(root_clip_layer->id());
   root_clip_layer->SetBounds(gfx::Size(50, 20));
   root_layer->SetBounds(gfx::Size(50, 100));
 
   layer_tree_host_->SetRootLayer(root_clip_layer);
   root_clip_layer->AddChild(root_layer);
   root_layer->AddChild(scrollbar_layer);
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(10, 20));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(0, 0, 10, 20));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
   scrollbar_layer->fake_scrollbar()->set_thumb_length(4);
   layer_tree_host_->UpdateLayers();
   LayerImpl* root_clip_layer_impl = nullptr;
   PaintedScrollbarLayerImpl* scrollbar_layer_impl = nullptr;
 
   // Thumb is at the edge of the scrollbar (should be inset to
   // the start of the track within the scrollbar layer's
   // position).
@@ skipping 732 matching lines @@
   TestScale(gfx::Rect(1240, 0, 15, 1333), 2.7754839f);
   TestScale(gfx::Rect(1240, 0, 15, 677), 2.46677136f);
 
   // Horizontal Scrollbars.
   TestScale(gfx::Rect(0, 1240, 1333, 15), 2.7754839f);
   TestScale(gfx::Rect(0, 1240, 677, 15), 2.46677136f);
 }
 
 }  // namespace
 }  // namespace cc