cc: Remove LayerImpl::children() calls after tree synchronization

cc/trees/layer_tree_host_unittest.cc

 // Copyright 2011 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/trees/layer_tree_host.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3755 matching lines @@
     inner_viewport_scroll_layer->SetScrollClipLayerId(
         inner_viewport_container_layer->id());
     inner_viewport_scroll_layer->SetIsContainerForFixedPositionLayers(true);
 
     root_layer_->AddChild(inner_viewport_container_layer);
     inner_viewport_container_layer->AddChild(overscroll_elasticity_layer);
     overscroll_elasticity_layer->AddChild(page_scale_layer);
     page_scale_layer->AddChild(inner_viewport_scroll_layer);
 
     scoped_refptr<Layer> content_layer = FakePictureLayer::Create(&client_);
+    content_layer_id_ = content_layer->id();
     content_layer->SetBounds(gfx::Size(10, 10));
     inner_viewport_scroll_layer->AddChild(content_layer);
 
     layer_tree_host()->SetRootLayer(root_layer_);
     layer_tree_host()->RegisterViewportLayers(
         overscroll_elasticity_layer, page_scale_layer,
         inner_viewport_scroll_layer, nullptr);
     LayerTreeHostTest::SetupTree();
     client_.set_bounds(content_layer->bounds());
   }
 
   void BeginTest() override { PostSetNeedsCommitToMainThread(); }
 
   void CommitCompleteOnThread(LayerTreeHostImpl* host_impl) override {
     if (host_impl->sync_tree()->source_frame_number() == 0) {
       scroll_elasticity_helper_ = host_impl->CreateScrollElasticityHelper();
     }
   }
 
   void DrawLayersOnThread(LayerTreeHostImpl* host_impl) override {
     num_draws_++;
     LayerImpl* content_layer_impl =
-        host_impl->active_tree()->InnerViewportScrollLayer()->children()[0];
+        host_impl->active_tree()->LayerById(content_layer_id_);
     gfx::Transform expected_draw_transform;
     switch (num_draws_) {
       case 1:
         // Initially, there's no overscroll.
         EXPECT_EQ(expected_draw_transform, content_layer_impl->DrawTransform());
 
         // Begin overscrolling. This should be reflected in the draw transform
         // the next time we draw.
         scroll_elasticity_helper_->SetStretchAmount(gfx::Vector2dF(5.f, 6.f));
         break;
@@ skipping 17 matching lines @@
         NOTREACHED();
     }
   }
 
   void AfterTest() override {}
 
  private:
   FakeContentLayerClient client_;
   scoped_refptr<Layer> root_layer_;
   ScrollElasticityHelper* scroll_elasticity_helper_;
+  int content_layer_id_;
   int num_draws_;
 };
 
 MULTI_THREAD_TEST_F(LayerTreeHostTestElasticOverscroll);
 
 class LayerTreeHostTestSetMemoryPolicyOnLostOutputSurface
     : public LayerTreeHostTest {
  protected:
   LayerTreeHostTestSetMemoryPolicyOnLostOutputSurface()
       : first_output_surface_memory_limit_(4321234),
@@ skipping 2452 matching lines @@
     EndTest();
   }
 
   void AfterTest() override {}
 };
 
 SINGLE_AND_MULTI_THREAD_TEST_F(LayerTreeHostTestPaintedDeviceScaleFactor);
 
 }  // namespace
 }  // namespace cc