Add tracing AutoOpenCloseEvent.

base/trace_event/persistent_async_event.cc

+// 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.
+
+#include "base/trace_event/persistent_async_event.h"
+
+#include "base/macros.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+
+namespace trace_event {
+
+PersistentAsyncEvent::PersistentAsyncEvent() : active_(false) {
+  id_ = static_cast<void*>(this);
+  base::trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
+}
+
+PersistentAsyncEvent::~PersistentAsyncEvent() {}

[benjhayden, 2016/08/30 04:17:23]

Do you need to RemoveEnabledStateObserver() to prevent use-after-free?

[alexandermont, 2016/08/30 19:01:22]

Done

+
+void PersistentAsyncEvent::Start() {
+  RecordStartEvent();
+  active_ = true;
+  start_time_ = base::TimeTicks::Now().ToInternalValue();
+}
+
+void PersistentAsyncEvent::Stop() {
+  RecordStopEvent();
+  active_ = false;
+}
+
+void PersistentAsyncEvent::OnTraceLogEnabled() {
+  if (active_) {
+    RecordStartEventWithTimestamp(start_time_);
+  }
+}
+
+void PersistentAsyncEvent::OnTraceLogDisabled() {
+  // TODO(alexandermont): How do we call RecordStopEvent before log disabled?

[benjhayden, 2016/08/30 04:17:23]

IIRC, primiano or oystein suggested trying to fix
TraceLog::SetDisabledWhileLocked() to call OnTraceLogDisabled() before setting
mode_ = DISABLED? If that would be more complicated than just moving a few lines
of code, then it can wait for another CL.

[alexandermont, 2016/08/30 19:01:22]

Done

+  if (active_) {
+    RecordStopEvent();
+  }
+}
+
+}  // namespace trace_event
+}  // namespace base