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

base/trace_event/trace_event_argument.cc

Index: base/trace_event/trace_event_argument.cc
diff --git a/base/trace_event/trace_event_argument.cc b/base/trace_event/trace_event_argument.cc
index 88b1879b8b3757071727d4a94103e8d10f304d05..e83aa7372480b56ef266bc96b8dca3a8c0d3bdc8 100644
--- a/base/trace_event/trace_event_argument.cc
+++ b/base/trace_event/trace_event_argument.cc
@@ -108,7 +108,7 @@ ListValue* TracedValue::GetCurrentArray() {
 
 void TracedValue::AppendAsTraceFormat(std::string* out) const {
   std::string tmp;
-  JSONWriter::Write(stack_.front(), &tmp);
+  JSONWriter::Write(*stack_.front(), &tmp);
   *out += tmp;
   DCHECK_EQ(1u, stack_.size()) << tmp;
 }