LayerDebugInfo should not clobber trace output

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index 97add3dae9a56166f57cee7f9791612d1cadd8df..6d1902a1e9505240f1c9bd469c7956b3cf2782c4 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -914,7 +914,9 @@ class LayerDebugInfo : public base::trace_event::ConvertableToTraceFormat {
   void AppendAsTraceFormat(std::string* out) const override {
     base::DictionaryValue dictionary;
     dictionary.SetString("layer_name", name_);
-    base::JSONWriter::Write(dictionary, out);
+    std::string tmp;
+    base::JSONWriter::Write(dictionary, &tmp);
+    out->append(tmp);
   }
 
  private: