Cleanup: Pass std::string as const reference if possible

chrome/browser/ui/views/message_center/web_notification_tray_browsertest.cc

 // Copyright 2013 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/ui/views/message_center/web_notification_tray.h"
 
 #include <set>
 
 #include "ash/root_window_controller.h"
 #include "ash/system/status_area_widget.h"
@@ skipping 28 matching lines @@
   WebNotificationTrayTest() {}
   ~WebNotificationTrayTest() override {}
 
   void TearDownOnMainThread() override {
     message_center::MessageCenter::Get()->RemoveAllNotifications(false);
   }
 
  protected:
   class TestNotificationDelegate : public ::NotificationDelegate {
    public:
-    explicit TestNotificationDelegate(std::string id) : id_(id) {}
+    explicit TestNotificationDelegate(const std::string& id) : id_(id) {}
     std::string id() const override { return id_; }
 
    private:
     ~TestNotificationDelegate() override {}
 
     std::string id_;
   };
 
   void AddNotification(const std::string& delegate_id,
                        const std::string& replace_id) {
@@ skipping 124 matching lines @@
   tray->message_center_tray_->HidePopupBubble();
   tray->message_center_tray_->ShowPopupBubble();
   EXPECT_TRUE(tray->message_center_tray_->popups_visible());
   EXPECT_EQ(notifications_to_add, message_center->NotificationCount());
   NotificationList::PopupNotifications popups =
       message_center->GetPopupNotifications();
   EXPECT_EQ(kMaxVisiblePopupNotifications, popups.size());
 }
 
 }  // namespace message_center