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

chrome/browser/sessions/session_restore_observer.h

Index: chrome/browser/sessions/session_restore_observer.h
diff --git a/chrome/browser/sessions/session_restore_observer.h b/chrome/browser/sessions/session_restore_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..efe853b951f2d9e3b3b3c05d4e82a8495e7f19e6
--- /dev/null
+++ b/chrome/browser/sessions/session_restore_observer.h
@@ -0,0 +1,26 @@
+// 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.
+
+#ifndef CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_
+#define CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_
+
+#include <unordered_set>
+
+// Observer of events during session restore.
+// The set of observers will be deleted when there is no observer.
+class SessionRestoreObserver {
+ public:
+  virtual void OnTabLoadingStarted() {}
+  virtual void OnTabLoadingEnded() {}
+
+  static std::unordered_set<SessionRestoreObserver*>* observers();

[lpy, 2017/06/23 22:35:57]

Let's use static base::ObserverList<SessionRestoreObserver>& observers();

[ducbui, 2017/06/23 23:30:14]

Done.

+
+  static void AddObserver(SessionRestoreObserver* observer);
+  static void RemoveObserver(SessionRestoreObserver* observer);
+
+ private:
+  static std::unordered_set<SessionRestoreObserver*>* observers_;

[lpy, 2017/06/23 22:35:57]

Let's use base::ObserverList<SessionRestoreObserver>*

[ducbui, 2017/06/23 23:30:14]

Done.

+};
+
+#endif  // CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_