cc: refactor of MathUtil::AddToTracedValue

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index db9b072c0d74209d9295fa75fc8622c1ac3983e3..526d456f62422217daa48c904ad0548f412e43a2 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3298,9 +3298,8 @@ void LayerTreeHostImpl::AsValueWithFrameInto(
     ActivationStateAsValueInto(state);
     state->EndDictionary();
   }
-  state->BeginDictionary("device_viewport_size");
-  MathUtil::AddToTracedValue(device_viewport_size_, state);
-  state->EndDictionary();
+  MathUtil::AddToTracedValue("device_viewport_size", device_viewport_size_,
+                             state);
 
   std::set<const Tile*> tiles;
   active_tree_->GetAllTilesForTracing(&tiles);