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

base/trace_event/trace_buffer.h

Index: base/trace_event/trace_buffer.h
diff --git a/base/trace_event/trace_buffer.h b/base/trace_event/trace_buffer.h
index 8107f322ebb3368fc8ea9d79f178cae2febcd982..4074120e4c9260be8ee1d3c29b3c09fdafa00261 100644
--- a/base/trace_event/trace_buffer.h
+++ b/base/trace_event/trace_buffer.h
@@ -44,7 +44,7 @@
 
  private:
   size_t next_free_;
-  scoped_ptr<TraceEventMemoryOverhead> cached_overhead_estimate_when_full_;
+  scoped_ptr<TraceEventMemoryOverhead> cached_overhead_estimate_;
   TraceEvent chunk_[kTraceBufferChunkSize];
   uint32 seq_;
 };