Convert JsonWriter::Write to taking a const ref for the in-param

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 0bf40594b49d519a242d450b1859190577331892..b2084af7d7e7eda92b2b6ca8bab1c140e7829c15 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3086,7 +3086,7 @@ std::string LayerTreeHostImpl::LayerTreeAsJson() const {
   if (active_tree_->root_layer()) {
     scoped_ptr<base::Value> json(active_tree_->root_layer()->LayerTreeAsJson());
     base::JSONWriter::WriteWithOptions(
-        json.get(), base::JSONWriter::OPTIONS_PRETTY_PRINT, &str);
+        *json, base::JSONWriter::OPTIONS_PRETTY_PRINT, &str);
   }
   return str;
 }