cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/test/layer_tree_json_parser_unittest.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/test/layer_tree_json_parser.h"
 
 #include <stddef.h>
 
 #include "cc/layers/layer.h"
 #include "cc/test/fake_impl_task_runner_provider.h"
@@ skipping 72 matching lines @@
 
   parent->SetPosition(gfx::PointF(25.f, 25.f));
 
   parent->AddChild(std::move(child));
   root_impl->AddChild(std::move(parent));
   tree->SetRootLayer(std::move(root_impl));
 
   std::string json = host_impl.LayerTreeAsJson();
   scoped_refptr<Layer> root = ParseTreeFromJson(json, NULL);
   ASSERT_TRUE(root.get());
-  EXPECT_TRUE(LayerTreesMatch(host_impl.RootLayer(), root.get()));
+  EXPECT_TRUE(
+      LayerTreesMatch(host_impl.active_tree()->root_layer(), root.get()));
 }
 
 TEST_F(LayerTreeJsonParserSanityCheck, EventHandlerRegions) {
   FakeImplTaskRunnerProvider task_runner_provider;
   TestSharedBitmapManager shared_bitmap_manager;
   TestTaskGraphRunner task_graph_runner;
   FakeLayerTreeHostImpl host_impl(&task_runner_provider, &shared_bitmap_manager,
                                   &task_graph_runner);
   LayerTreeImpl* tree = host_impl.active_tree();
 
   std::unique_ptr<LayerImpl> root_impl(LayerImpl::Create(tree, 1));
   std::unique_ptr<LayerImpl> touch_layer(LayerImpl::Create(tree, 2));
 
   root_impl->SetBounds(gfx::Size(100, 100));
   touch_layer->SetBounds(gfx::Size(50, 50));
 
   Region touch_region;
   touch_region.Union(gfx::Rect(10, 10, 20, 30));
   touch_region.Union(gfx::Rect(40, 10, 20, 20));
   touch_layer->SetTouchEventHandlerRegion(touch_region);
 
   root_impl->AddChild(std::move(touch_layer));
   tree->SetRootLayer(std::move(root_impl));
 
   std::string json = host_impl.LayerTreeAsJson();
   scoped_refptr<Layer> root = ParseTreeFromJson(json, NULL);
   ASSERT_TRUE(root.get());
-  EXPECT_TRUE(LayerTreesMatch(host_impl.RootLayer(), root.get()));
+  EXPECT_TRUE(
+      LayerTreesMatch(host_impl.active_tree()->root_layer(), root.get()));
 }
 
 }  // namespace cc