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

chrome/browser/sessions/session_restore_observer.h

+// 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_
+
+// Observer of events during session restore.
+class SessionRestoreObserver {
+ public:
+  // OnSessionRestoreStartedLoadingTabs is triggered when the browser starts
+  // loading tabs in session restore.
+  virtual void OnSessionRestoreStartedLoadingTabs() {}
+
+  // OnSessionRestoreFinishedLoadingTabs is triggered when the browser finishes
+  // loading tabs in session restore. In case of memory pressure, not all
+  // WebContents may have been created (i.e., some are deferred).
+  virtual void OnSessionRestoreFinishedLoadingTabs() {}
+};
+
+#endif  // CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_