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

base/trace_event/trace_event_memory_overhead.cc

Index: base/trace_event/trace_event_memory_overhead.cc
diff --git a/base/trace_event/trace_event_memory_overhead.cc b/base/trace_event/trace_event_memory_overhead.cc
index 0cc3d59890c57884e8fe035d1afba7f6600528b1..5ece315c00f30485ee0dce9c997b83de6007f3ee 100644
--- a/base/trace_event/trace_event_memory_overhead.cc
+++ b/base/trace_event/trace_event_memory_overhead.cc
@@ -127,6 +127,13 @@ void TraceEventMemoryOverhead::AddSelf() {
   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,