Add tracing AutoOpenCloseEvent.

base/trace_event/trace_log.cc

Index: base/trace_event/trace_log.cc
diff --git a/base/trace_event/trace_log.cc b/base/trace_event/trace_log.cc
index 1a9af140551254bf6111c54ed4fb2a55d2e50121..451eba08b2cce92cbfc1a5ba811a11e6558ca9a9 100644
--- a/base/trace_event/trace_log.cc
+++ b/base/trace_event/trace_log.cc
@@ -834,7 +834,7 @@ void TraceLog::SetDisabledWhileLocked() {
     return;
   }
 
-  mode_ = DISABLED;
+  //mode_ = DISABLED;
 
   if (sampling_thread_) {
     // Stop the sampling thread.
@@ -867,6 +867,7 @@ void TraceLog::SetDisabledWhileLocked() {
     AutoUnlock unlock(lock_);
     for (EnabledStateObserver* observer : observer_list)
       observer->OnTraceLogDisabled();
+    mode_ = DISABLED;

[Primiano Tucci (use gerrit), 2016/09/14 09:38:54]

???

[alexandermont, 2016/09/14 20:14:57]

Reverted this change. Originally this was there because we were going to
effectively change OnTraceLogDisabled to be called before the trace log is
disabled; however it turned out that that made some unit tests fail. Now we're
going to change it to use OnBeforeTraceLogDisabled (unless we're going to change
it to something else...)

     for (const auto& it : observer_map) {
       it.second.task_runner->PostTask(
           FROM_HERE, Bind(&AsyncEnabledStateObserver::OnTraceLogDisabled,