Add option to export tracing events to ETW.

base/trace_event/trace_event.gypi

Index: base/trace_event/trace_event.gypi
diff --git a/base/trace_event/trace_event.gypi b/base/trace_event/trace_event.gypi
index ca6c0765992af9367011ebf40f7e274a06e915d1..d541743f4035d90e842a0e57111dbe3d6afe2d85 100644
--- a/base/trace_event/trace_event.gypi
+++ b/base/trace_event/trace_event.gypi
@@ -25,6 +25,8 @@
       'trace_event/trace_event_android.cc',
       'trace_event/trace_event_argument.cc',
       'trace_event/trace_event_argument.h',
+      'trace_event/trace_event_etw_export_win.cc',
+      'trace_event/trace_event_etw_export_win.h',
       'trace_event/trace_event_impl.cc',
       'trace_event/trace_event_impl.h',
       'trace_event/trace_event_impl_constants.cc',
@@ -48,6 +50,6 @@
       'trace_event/trace_event_system_stats_monitor_unittest.cc',
       'trace_event/trace_event_unittest.cc',
       'trace_event/trace_event_win_unittest.cc',
-    ],
+    ],
   },
 }