cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/layer_tree_impl_unittest.cc

 // Copyright 2014 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_impl.h"
 
 #include "cc/layers/heads_up_display_layer_impl.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/solid_color_scrollbar_layer_impl.h"
 #include "cc/test/fake_impl_task_runner_provider.h"
@@ skipping 117 matching lines @@
   SetLayerPropertiesForTesting(root.get(), identity_matrix, transform_origin,
                                position, bounds, true, false, true);
   root->SetDrawsContent(true);
 
   host_impl->SetViewportSize(root->bounds());
   host_impl->active_tree()->SetRootLayer(std::move(root));
   host_impl->UpdateNumChildrenAndDrawPropertiesForActiveTree();
   EXPECT_EQ(
       gfx::RectF(gfx::SizeF(bounds)),
       host_impl->active_tree()->property_trees()->clip_tree.ViewportClip());
-  EXPECT_EQ(gfx::Rect(bounds), host_impl->RootLayer()->visible_layer_rect());
+  EXPECT_EQ(gfx::Rect(bounds),
+            host_impl->active_tree()->root_layer()->visible_layer_rect());
 
   gfx::Size new_bounds(50, 50);
   host_impl->SetViewportSize(new_bounds);
   gfx::PointF test_point(51.f, 51.f);
   host_impl->active_tree()->FindLayerThatIsHitByPoint(test_point);
   EXPECT_EQ(
       gfx::RectF(gfx::SizeF(new_bounds)),
       host_impl->active_tree()->property_trees()->clip_tree.ViewportClip());
   EXPECT_EQ(gfx::Rect(new_bounds),
-            host_impl->RootLayer()->visible_layer_rect());
+            host_impl->active_tree()->root_layer()->visible_layer_rect());
 }
 
 TEST_F(LayerTreeImplTest, HitTestingForSingleLayerAndHud) {
   std::unique_ptr<LayerImpl> root =
       LayerImpl::Create(host_impl().active_tree(), 12345);
   std::unique_ptr<HeadsUpDisplayLayerImpl> hud =
       HeadsUpDisplayLayerImpl::Create(host_impl().active_tree(), 11111);
 
   gfx::Transform identity_matrix;
   gfx::Point3F transform_origin;
@@ skipping 2387 matching lines @@
   gfx::PointF test_point = gfx::PointF(1.f, 1.f);
   LayerImpl* result_layer =
       host_impl().active_tree()->FindLayerThatIsHitByPoint(test_point);
 
   CHECK(result_layer);
   EXPECT_EQ(2, result_layer->id());
 }
 
 }  // namespace
 }  // namespace cc