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

base/trace_event/heap_profiler_allocation_register.cc

Index: base/trace_event/heap_profiler_allocation_register.cc
diff --git a/base/trace_event/heap_profiler_allocation_register.cc b/base/trace_event/heap_profiler_allocation_register.cc
index b9f440adb69d5a131fdc14fa2cde7a2f1da6466d..bd06bd234724ffd88406e422c16bccedc75d9d61 100644
--- a/base/trace_event/heap_profiler_allocation_register.cc
+++ b/base/trace_event/heap_profiler_allocation_register.cc
@@ -158,7 +158,8 @@ void AllocationRegister::EstimateTraceMemoryOverhead(
   size_t resident = sizeof(AllocationRegister) +
                     allocations_.EstimateUsedMemory() +
                     backtraces_.EstimateUsedMemory();
-  overhead->Add("AllocationRegister", allocated, resident);
+  overhead->Add(TraceEventMemoryOverhead::kHeapProfilerAllocationRegister,
+                allocated, resident);
 }
 
 AllocationRegister::BacktraceMap::KVIndex AllocationRegister::InsertBacktrace(