tracing: Simplify TraceEventMemoryOverhead, use an enum insted of a map

base/trace_event/trace_event_argument.cc

 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/trace_event/trace_event_argument.h"
 
 #include <stdint.h>
 
 #include <utility>
 
@@ skipping 447 matching lines @@
   // TODO(primiano): this could be smarter, skip the ToBaseValue encoding and
   // produce the JSON on its own. This will require refactoring JSONWriter
   // to decouple the base::Value traversal from the JSON writing bits
   std::string tmp;
   JSONWriter::Write(*ToBaseValue(), &tmp);
   *out += tmp;
 }
 
 void TracedValue::EstimateTraceMemoryOverhead(
     TraceEventMemoryOverhead* overhead) {
-  overhead->Add("TracedValue",
+  overhead->Add(TraceEventMemoryOverhead::kTracedValue,
                 /* allocated size */
                 pickle_.GetTotalAllocatedSize(),
                 /* resident size */
                 pickle_.size());
 }
 
 }  // namespace trace_event
 }  // namespace base