cc: Remove LayerTreeImpl::root_layer usage outside tests

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 3203 matching lines @@
         // Root is swapped here.
         // Clear the expected push properties the tree will be rebuilt.
         expected_push_properties_root_impl_ = 0;
         expected_push_properties_child_impl_ = 0;
         expected_push_properties_grandchild_impl_ = 0;
         expected_push_properties_child2_impl_ = 0;
         expected_push_properties_grandchild2_impl_ = 0;
 
         // Make sure the new root is pushed.
         EXPECT_EQ(1u, static_cast<PushPropertiesCountingLayerImpl*>(
-                host_impl->RootLayer())->push_properties_count());
+                          host_impl->active_tree()->root_layer())
+                          ->push_properties_count());
         return;
       case 4:
         // Root is swapped back all of the layers in the tree get pushed.
         ++expected_push_properties_root_impl_;
         ++expected_push_properties_child_impl_;
         ++expected_push_properties_grandchild_impl_;
         ++expected_push_properties_child2_impl_;
         ++expected_push_properties_grandchild2_impl_;
         break;
       case 5:
@@ skipping 84 matching lines @@
 
     PushPropertiesCountingLayerImpl* root_impl_ = NULL;
     PushPropertiesCountingLayerImpl* child_impl_ = NULL;
     PushPropertiesCountingLayerImpl* child2_impl_ = NULL;
     PushPropertiesCountingLayerImpl* grandchild_impl_ = NULL;
     PushPropertiesCountingLayerImpl* leaf_always_pushing_layer_impl_ = NULL;
 
     // Pull the layers that we need from the tree assuming the same structure
     // as LayerTreeHostTestLayersPushProperties
     root_impl_ = static_cast<PushPropertiesCountingLayerImpl*>(
-        host_impl->RootLayer());
+        host_impl->active_tree()->root_layer());
 
     LayerTreeImpl* impl = root_impl_->layer_tree_impl();
     if (impl->LayerById(child_->id())) {
       child_impl_ = static_cast<PushPropertiesCountingLayerImpl*>(
           impl->LayerById(child_->id()));
 
       if (impl->LayerById(grandchild_->id()))
         grandchild_impl_ = static_cast<PushPropertiesCountingLayerImpl*>(
             impl->LayerById(grandchild_->id()));
     }
@@ skipping 3328 matching lines @@
     EndTest();
   }
 
   void AfterTest() override {}
 };
 
 SINGLE_AND_MULTI_THREAD_TEST_F(LayerTreeHostTestPaintedDeviceScaleFactor);
 
 }  // namespace
 }  // namespace cc