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

base/trace_event/trace_event_impl.h

Index: base/trace_event/trace_event_impl.h
diff --git a/base/trace_event/trace_event_impl.h b/base/trace_event/trace_event_impl.h
index 93212558aa7a451eca89e9fb112f450a765db250..4274f807d2ae029da263dcd342377a2813808f7c 100644
--- a/base/trace_event/trace_event_impl.h
+++ b/base/trace_event/trace_event_impl.h
@@ -176,7 +176,6 @@ class BASE_EXPORT TraceEvent {
   TimeDelta thread_duration_;
   // id_ can be used to store phase-specific data.
   unsigned long long id_;
-  scoped_ptr<TraceEventMemoryOverhead> cached_memory_overhead_estimate_;
   // context_id_ is used to store context information.
   unsigned long long context_id_;
   TraceValue arg_values_[kTraceMaxNumArgs];
@@ -224,7 +223,7 @@ class BASE_EXPORT TraceBufferChunk {
 
  private:
   size_t next_free_;
-  scoped_ptr<TraceEventMemoryOverhead> cached_overhead_estimate_when_full_;
+  scoped_ptr<TraceEventMemoryOverhead> cached_overhead_estimate_;
   TraceEvent chunk_[kTraceBufferChunkSize];
   uint32 seq_;
 };