[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..3eee10ed26cb80c840309aeff9c572084a198d81
--- /dev/null
+++ b/chrome/browser/sessions/session_restore_observer.h
@@ -0,0 +1,20 @@
+// 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.

[sky, 2017/06/27 00:11:53]

Please also document that in the case of memory pressure not all WebContents may
have been created.

[ducbui, 2017/06/27 00:45:03]

Done.

+  virtual void OnSessionRestoreFinishedLoadingTabs() {}
+};
+
+#endif  // CHROME_BROWSER_SESSIONS_SESSION_RESTORE_OBSERVER_H_