Add tracing AutoOpenCloseEvent.

base/trace_event/auto_open_close_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_AUTO_OPEN_CLOSE_EVENT_H_
+#define BASE_AUTO_OPEN_CLOSE_EVENT_H_
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+namespace trace_event {
+
+/**

[Primiano Tucci (use gerrit), 2016/09/22 10:05:49]

Nit: in general I see always // style comments. The style guide  says you can
use both as long you are consistent, which is not the case here :)
More importantly all the code in //base seems to use //, so let's just stick
with that.

+ * Class for tracing events that support "auto-opening" and "auto-closing".
+ * "auto-opening" = if the trace event is started (call Begin() before
+ * tracing is started,the trace event will be opened, with the start time
+ * being the time that the trace event was actually started.
+ * "auto-closing" = if the trace event is started but not ended by the time
+ * tracing ends, then the trace event will be automatically closed at the
+ * end of tracing
+ */
+class BASE_EXPORT AutoOpenCloseEvent
+    : public TraceLog::AsyncEnabledStateObserver {
+ public:
+  enum Type {
+    ASYNC
+  };
+
+  // As in the rest of the tracing macros, the const char* arguments here
+  // must be pointers to indefinitely lived strings (e.g. hard-coded string
+  // literals are okay, but not strings created by c_str())
+  AutoOpenCloseEvent(Type type, const char* category, const char* event_name);
+  ~AutoOpenCloseEvent() override;
+
+  void Begin();
+  void End();
+
+  // AsyncEnabledStateObserver implementation
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+ private:
+  const char* const category_;
+  const char* const event_name_;
+  base::TimeTicks start_time_;
+  base::ThreadChecker thread_checker_;
+  WeakPtrFactory<AutoOpenCloseEvent> weak_factory_;
+  DISALLOW_COPY_AND_ASSIGN(AutoOpenCloseEvent);
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_AUTO_OPEN_CLOSE_EVENT_H_