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

chrome/browser/sessions/session_restore.cc

 // Copyright 2012 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.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <list>
@@ skipping 847 matching lines @@
   for (auto* session_restorer : *active_session_restorers) {
     if (session_restorer->profile() == profile) {
       session_restorer->AddURLsToOpen(urls);
       return;
     }
   }
   NOTREACHED() << "Failed to add urls to open for session restore";
 }
 
 // static
+void SessionRestore::AddObserver(SessionRestoreObserver* observer) {
+  observers().AddObserver(observer);
+}
+
+// static
+void SessionRestore::RemoveObserver(SessionRestoreObserver* observer) {
+  observers().RemoveObserver(observer);
+}
+
+// static
 base::CallbackList<void(int)>*
     SessionRestore::on_session_restored_callbacks_ = nullptr;
+
+// static
+base::ObserverList<SessionRestoreObserver>* SessionRestore::observers_ =
+    nullptr;