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

base/trace_event/heap_profiler_stack_frame_deduplicator.cc

 // Copyright 2015 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/heap_profiler_stack_frame_deduplicator.h"
 
 #include <inttypes.h>
 #include <stddef.h>
 
 #include <string>
@@ skipping 105 matching lines @@
       out->append(",");
   }
 
   out->append("}");  // End the |stackFrames| dictionary.
 }
 
 void StackFrameDeduplicator::EstimateTraceMemoryOverhead(
     TraceEventMemoryOverhead* overhead) {
   size_t memory_usage =
       EstimateMemoryUsage(frames_) + EstimateMemoryUsage(roots_);
-  overhead->Add("StackFrameDeduplicator",
+  overhead->Add(TraceEventMemoryOverhead::kHeapProfilerStackFrameDeduplicator,
                 sizeof(StackFrameDeduplicator) + memory_usage);
 }
 
 }  // namespace trace_event
 }  // namespace base