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

base/trace_event/trace_event_memory_overhead.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/trace_event_memory_overhead.h"
 
 #include <algorithm>
 
 #include "base/bits.h"
 #include "base/memory/ref_counted_memory.h"
@@ skipping 104 matching lines @@
 
 void TraceEventMemoryOverhead::AddSelf() {
   size_t estimated_size = sizeof(*this);
   // If the SmallMap did overflow its static capacity, its elements will be
   // allocated on the heap and have to be accounted separately.
   if (allocated_objects_.UsingFullMap())
     estimated_size += sizeof(map_type::value_type) * allocated_objects_.size();
   Add("TraceEventMemoryOverhead", estimated_size);
 }
 
+size_t TraceEventMemoryOverhead::GetCount(const char* object_type) const {
+  const auto& it = allocated_objects_.find(object_type);
+  if (it == allocated_objects_.end())
+    return 0u;
+  return it->second.count;
+}
+
 void TraceEventMemoryOverhead::Update(const TraceEventMemoryOverhead& other) {
   for (const auto& it : other.allocated_objects_) {
     AddOrCreateInternal(it.first, it.second.count,
                         it.second.allocated_size_in_bytes,
                         it.second.resident_size_in_bytes);
   }
 }
 
 void TraceEventMemoryOverhead::DumpInto(const char* base_name,
                                         ProcessMemoryDump* pmd) const {
   for (const auto& it : allocated_objects_) {
     std::string dump_name = StringPrintf("%s/%s", base_name, it.first);
     MemoryAllocatorDump* mad = pmd->CreateAllocatorDump(dump_name);
     mad->AddScalar(MemoryAllocatorDump::kNameSize,
                    MemoryAllocatorDump::kUnitsBytes,
                    it.second.allocated_size_in_bytes);
     mad->AddScalar("resident_size", MemoryAllocatorDump::kUnitsBytes,
                    it.second.resident_size_in_bytes);
     mad->AddScalar(MemoryAllocatorDump::kNameObjectsCount,
                    MemoryAllocatorDump::kUnitsObjects, it.second.count);
   }
 }
 
 }  // namespace trace_event
 }  // namespace base