[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..3d200c2456f53af788d98c861e4aeb0e94d6d45f
--- /dev/null
+++ b/chrome/browser/sessions/session_restore_observer.h
@@ -0,0 +1,27 @@
+// 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>
+
+#include "base/observer_list.h"
+
+// Observer of events during session restore.
+class SessionRestoreObserver {
+ public:
+  virtual void OnTabLoadingStarted() {}

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

Clearly document at what point these functions are called.

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

Given these functions are on the SessionRestore interface I think it better to
name them something like OnSessionRestoreStartedLoadingTabs and
OnSessionRestoreFinishedLoadingTabs.

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

Please add test coverage that these functions are called.

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

Done.

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

Done.

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

Done.

+  virtual void OnTabLoadingEnded() {}
+
+  static base::ObserverList<SessionRestoreObserver>& observers();
+
+  static void AddObserver(SessionRestoreObserver* observer);

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

Move these functions onto SessionRestore.

Also, the callback in SessionRestore should be replaced by your observer, but
that can be done separately.

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

Done.
Do you mean the callback in SessionRestore::RegisterOnSessionRestoredCallback()?
Do you want me to include the replacement in this CL?

[sky, 2017/06/26 21:38:08]

Exactly.
That would be great, but I'm ok if you want to push that to a follow on cl.

+  static void RemoveObserver(SessionRestoreObserver* observer);
+
+ private:
+  static base::ObserverList<SessionRestoreObserver>* observers_;
+};
+
+#endif  // CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_