[TabMetrics] Add signals that mark the start and end of session restore.

chrome/browser/sessions/session_restore_observer.cc

+// Copyright 2017 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 "chrome/browser/sessions/session_restore_observer.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+
+// static
+base::ObserverList<SessionRestoreObserver>&
+SessionRestoreObserver::observers() {
+  if (!observers_)
+    observers_ = new base::ObserverList<SessionRestoreObserver>();
+  return *observers_;
+}
+
+// static
+void SessionRestoreObserver::AddObserver(SessionRestoreObserver* observer) {
+  DCHECK(!observers().HasObserver(observer)) << "Duplicate registration";

[sky, 2017/06/26 15:34:10]

There is no need for this DCHECK, it's in ObserverList.

[ducbui, 2017/06/26 19:59:26]

Done.

+  observers().AddObserver(observer);
+}
+
+// static
+void SessionRestoreObserver::RemoveObserver(SessionRestoreObserver* observer) {
+  DCHECK(observers().HasObserver(observer));

[sky, 2017/06/26 15:34:11]

Remove DCHECK.

[ducbui, 2017/06/26 19:59:26]

Done.

+  observers().RemoveObserver(observer);
+}
+
+// static
+base::ObserverList<SessionRestoreObserver>* SessionRestoreObserver::observers_ =
+    nullptr;