Add tracing AutoOpenCloseEvent.

base/trace_event/persistent_async_event.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_
+#define BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_
+
+#include "base/macros.h"
+#include "base/threading/thread_checker.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"

[Primiano Tucci (use gerrit), 2016/09/15 10:08:29]

don't think you need to include trace_event here anymore

[Primiano Tucci (use gerrit), 2016/09/15 10:08:29]

IWYU (include what you use): add an include base/memory/weak_ptr.h

[alexandermont, 2016/09/15 21:05:13]

Done. But you do still need to include trace_event: it gives compiler errors
otherwise.

+
+namespace base {
+namespace trace_event {
+
+class BASE_EXPORT PersistentAsyncEvent

[Primiano Tucci (use gerrit), 2016/09/15 10:08:29]

Add a comment explaining what this class does and which problem solves.
I think the thing you want to clarify is that if you call begin() and end()
before tracing start you never see the event. but on the other end you are 
guaranteed that an event that is started before tracing will be there.

[alexandermont, 2016/09/15 21:05:13]

Done

+    : public trace_event::TraceLog::AsyncEnabledStateObserver {

[Primiano Tucci (use gerrit), 2016/09/15 10:08:30]

you don't need trace_event:: here as you are already in base::trace_event

[alexandermont, 2016/09/15 21:05:13]

Done

+ public:
+  enum Type {
+    ASYNC
+  };
+
+  PersistentAsyncEvent(PersistentAsyncEvent::Type type,

[Primiano Tucci (use gerrit), 2016/09/15 10:08:29]

you don't need PersistentAsyncEvent:: here I believe, just Type

[Primiano Tucci (use gerrit), 2016/09/15 10:08:30]

Can you add a comment saying
// As in the rest of the tracing macros, the const char* arguments here must be
pointers to indefinitely lived strings.

to remove the temptation to pass c_str() args

[alexandermont, 2016/09/15 21:05:13]

Done

+      const char* category, const char* event_name);
+  ~PersistentAsyncEvent() override;
+
+  void Begin();
+  void End();
+
+  // EnabledStateObserver implementation

[Primiano Tucci (use gerrit), 2016/09/15 10:08:30]

nit, +Async. This is now extending AsyncEnabledStateObserver 
(It was a trap, I first asked you to add a comment here and then to use
AsyncEnabledStateObserver :P)

[alexandermont, 2016/09/15 21:05:13]

Done

+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+  // TODO(alexandermont): Add in auto-close in OnBeforeTraceLogDisabled once the
+  // OnBeforeTraceLogDisabled CL lands.
+
+ private:
+  bool active_;
+  const char* const category_;
+  const char* const event_name_;
+  base::TimeTicks start_time_;
+  base::ThreadChecker thread_checker_;
+  WeakPtrFactory<PersistentAsyncEvent> weak_factory_;
+  DISALLOW_COPY_AND_ASSIGN(PersistentAsyncEvent);

[Primiano Tucci (use gerrit), 2016/09/15 10:08:30]

nit: add a blankline between fields and DISALLOW...

[alexandermont, 2016/09/15 21:05:13]

Done

+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_