Reland of [tracing] Fix, simplify and speed up accounting of TraceEvent memory overhead

base/trace_event/trace_event_memory_overhead.h

Index: base/trace_event/trace_event_memory_overhead.h
diff --git a/base/trace_event/trace_event_memory_overhead.h b/base/trace_event/trace_event_memory_overhead.h
index 8ecf12dc969eb4f0dc26516061450cff5d229509..25853d11d1284d7658085fe6125e72b6fcd97d8c 100644
--- a/base/trace_event/trace_event_memory_overhead.h
+++ b/base/trace_event/trace_event_memory_overhead.h
@@ -43,6 +43,9 @@
   // this TraceEventMemoryOverhead instance itself.
   void AddSelf();
 
+  // Retrieves the count, that is, the count of Add*(|object_type|, ...) calls.
+  size_t GetCount(const char* object_type) const;
+
   // Adds up and merges all the values from |other| to this instance.
   void Update(const TraceEventMemoryOverhead& other);