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

chrome/browser/sessions/session_restore.cc

Index: chrome/browser/sessions/session_restore.cc
diff --git a/chrome/browser/sessions/session_restore.cc b/chrome/browser/sessions/session_restore.cc
index 2818f7fe7e64ab24f50f5bc6b64e54eef9ba59d7..49831a27f087a8f5ec11c6b7b8b9b58499f6cffd 100644
--- a/chrome/browser/sessions/session_restore.cc
+++ b/chrome/browser/sessions/session_restore.cc
@@ -864,6 +864,20 @@ void SessionRestore::AddURLsToOpen(const Profile* profile,
   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;