cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/layer_tree_host_unittest.cc

Index: cc/trees/layer_tree_host_unittest.cc
diff --git a/cc/trees/layer_tree_host_unittest.cc b/cc/trees/layer_tree_host_unittest.cc
index 80635f02a7be8dcc42eca0b4a03c674d48b38740..c7b67c07b68ba71492d598902f0ec4691443f9f4 100644
--- a/cc/trees/layer_tree_host_unittest.cc
+++ b/cc/trees/layer_tree_host_unittest.cc
@@ -3221,7 +3221,8 @@ class LayerTreeHostTestImplLayersPushProperties
 
         // 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.
@@ -3326,7 +3327,7 @@ class LayerTreeHostTestImplLayersPushProperties
     // 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())) {