Reduce size of libevent notification trace event

This event is extremely frequent and contributes heavily to trace
size bloat, which is negatively affecting a lot of the testing
infrastructure. Reducing the size of the event and removing the args
which are not useful in this case.

R=gab@chromium.org
BUG=902064

Change-Id: Id7d0ecba3ed084f5ef30d7a29554b14c6a68b963
Reviewed-on: https://chromium-review.googlesource.com/c/1330067
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608958}

base/message_loop/message_pump_libevent.cc

@@ -306,9 +306,7 @@ void MessagePumpLibevent::OnLibeventNotification(int fd,
                                                  void* context) {
   FdWatchController* controller = static_cast<FdWatchController*>(context);
   DCHECK(controller);
-  TRACE_EVENT2("toplevel", "MessagePumpLibevent::OnLibeventNotification",
-               "src_file", controller->created_from_location().file_name(),
-               "src_func", controller->created_from_location().function_name());
+  TRACE_EVENT0("toplevel", "OnLibevent");
   TRACE_HEAP_PROFILER_API_SCOPED_TASK_EXECUTION heap_profiler_scope(
       controller->created_from_location().file_name());