Update from https://crrev.com/308996

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index c80826c881ee6bf8af6d9eaf44ffa353dbffd46b..77a9ada6bfa661f12cd13c97c4df67d08d6725de 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -538,10 +538,11 @@ class BASE_EXPORT TraceLog {
   // Due to the implementation of thread-local buffers, flush can't be
   // done when tracing is enabled. If called when tracing is enabled, the
   // callback will be called directly with (empty_string, false) to indicate
-  // the end of this unsuccessful flush.
+  // the end of this unsuccessful flush. Flush does the serialization
+  // on the same thread if the caller doesn't set use_worker_thread explicitly.
   typedef base::Callback<void(const scoped_refptr<base::RefCountedString>&,
                               bool has_more_events)> OutputCallback;
-  void Flush(const OutputCallback& cb);
+  void Flush(const OutputCallback& cb, bool use_worker_thread = false);
   void FlushButLeaveBufferIntact(const OutputCallback& flush_output_callback);
 
   // Called by TRACE_EVENT* macros, don't call this directly.
@@ -710,7 +711,9 @@ class BASE_EXPORT TraceLog {
   // |generation| is used in the following callbacks to check if the callback
   // is called for the flush of the current |logged_events_|.
   void FlushCurrentThread(int generation);
-  void ConvertTraceEventsToTraceFormat(scoped_ptr<TraceBuffer> logged_events,
+  // Usually it runs on a different thread.
+  static void ConvertTraceEventsToTraceFormat(
+      scoped_ptr<TraceBuffer> logged_events,
       const TraceLog::OutputCallback& flush_output_callback);
   void FinishFlush(int generation);
   void OnFlushTimeout(int generation);
@@ -804,6 +807,7 @@ class BASE_EXPORT TraceLog {
   OutputCallback flush_output_callback_;
   scoped_refptr<MessageLoopProxy> flush_message_loop_proxy_;
   subtle::AtomicWord generation_;
+  bool use_worker_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceLog);
 };