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

base/trace_event/trace_event_memory_overhead.h

 // 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.
 
 #ifndef BASE_TRACE_EVENT_TRACE_EVENT_MEMORY_OVERHEAD_H_
 #define BASE_TRACE_EVENT_TRACE_EVENT_MEMORY_OVERHEAD_H_
 
 #include "base/base_export.h"
 #include "base/containers/hash_tables.h"
 #include "base/containers/small_map.h"
@@ skipping 25 matching lines @@
 
   // Specialized profiling functions for commonly used object types.
   void AddString(const std::string& str);
   void AddValue(const Value& value);
   void AddRefCountedString(const RefCountedString& str);
 
   // Call this after all the Add* methods above to account the memory used by
   // 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);
 
   void DumpInto(const char* base_name, ProcessMemoryDump* pmd) const;
 
  private:
   struct ObjectCountAndSize {
     size_t count;
     size_t allocated_size_in_bytes;
     size_t resident_size_in_bytes;
   };
   using map_type = SmallMap<hash_map<const char*, ObjectCountAndSize>, 16>;
   map_type allocated_objects_;
 
   void AddOrCreateInternal(const char* object_type,
                            size_t count,
                            size_t allocated_size_in_bytes,
                            size_t resident_size_in_bytes);
 
   DISALLOW_COPY_AND_ASSIGN(TraceEventMemoryOverhead);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_MEMORY_OVERHEAD_H_