Fix tabs duplication when restoring last closed window.

components/sessions/core/tab_restore_service_helper.cc

Index: components/sessions/core/tab_restore_service_helper.cc
diff --git a/components/sessions/core/tab_restore_service_helper.cc b/components/sessions/core/tab_restore_service_helper.cc
index 1390ba1f6edade9ef1face23552d615bbefb4300..05094d2f7c9e762bd8c199d0a025866184633168 100644
--- a/components/sessions/core/tab_restore_service_helper.cc
+++ b/components/sessions/core/tab_restore_service_helper.cc
@@ -260,6 +260,10 @@ std::vector<LiveTab*> TabRestoreServiceHelper::RestoreEntryById(
   return live_tabs;
 }
 
+bool TabRestoreServiceHelper::IsRestoring() const {
+  return restoring_;
+}
+
 void TabRestoreServiceHelper::NotifyTabsChanged() {
   FOR_EACH_OBSERVER(TabRestoreServiceObserver, observer_list_,
                     TabRestoreServiceChanged(tab_restore_service_));