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() {
+  base::trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
+}
+
+void PersistentAsyncEvent::Begin() {
+  RecordBeginEvent();
+  active_ = true;
+  start_time_ = base::TimeTicks::Now();
+}
+
+void PersistentAsyncEvent::End() {
+  RecordEndEvent();
+  active_ = false;
+}
+
+void PersistentAsyncEvent::OnTraceLogEnabled() {
+  if (active_)
+    RecordBeginEventWithTimestamp(start_time_);
+}
+
+void PersistentAsyncEvent::OnTraceLogDisabled() {

[fmeawad, 2016/09/02 19:43:25]

I think we decided that we are going to leave the auto-close for post processing
and I believe that this will fail because we do not yet have
onBeforeTraceLogDisabled?

[benjhayden, 2016/09/02 20:57:45]

Fadi, the latest patch changes TraceLog::SetDisabledWhileLocked() so that
OnTraceLogDisabled is called before the TraceLog is disabled, so this should
work IIUC, at least for synchronous observers though not async observers.

+  // TODO(alexandermont): How do we call RecordStopEvent before log disabled?
+  if (active_)
+    RecordEndEvent();
+}
+
+}  // namespace trace_event
+}  // namespace base