Use base::BindOnce in notifications code

https://chromium-review.googlesource.com/c/chromium/src/+/1418170

atom/browser/notifications/platform_notification_service.cc

@@ -122,7 +122,7 @@ void PlatformNotificationService::CloseNotification(
 
 void PlatformNotificationService::GetDisplayedNotifications(
     content::BrowserContext* browser_context,
-    const DisplayedNotificationsCallback& callback) {}
+    DisplayedNotificationsCallback callback) {}
 
 int64_t PlatformNotificationService::ReadNextPersistentNotificationId(
     content::BrowserContext* browser_context) {

atom/browser/notifications/platform_notification_service.h

@@ -43,7 +43,7 @@ class PlatformNotificationService
                          const std::string& notification_id) override;
   void GetDisplayedNotifications(
       content::BrowserContext* browser_context,
-      const DisplayedNotificationsCallback& callback) override;
+      DisplayedNotificationsCallback callback) override;
   int64_t ReadNextPersistentNotificationId(
       content::BrowserContext* browser_context) override;
   void RecordNotificationUkmEvent(