Add ENABLE_NATIVE_NOTIFICATIONS guard for features::kNativeNotifications

When the build flag enable_native_notifications = false, a build error
happens. This CL adds ENABLE_NATIVE_NOTIFICATIONS guard to fix
this problem.

Bug: None
Change-Id: I2ab67b9f41898404eb3096f25c6dbd9ac31b7255
Reviewed-on: https://chromium-review.googlesource.com/1012728Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Joone Hur <joone.hur@intel.com>
Cr-Commit-Position: refs/heads/master@{#550950}

chrome/browser/notifications/notification_display_service_impl.cc

@@ -283,10 +283,12 @@ void NotificationDisplayServiceImpl::ProfileLoadedCallback(
 void NotificationDisplayServiceImpl::OnNotificationPlatformBridgeReady(
     bool success) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+#if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
   if (base::FeatureList::IsEnabled(features::kNativeNotifications)) {
     UMA_HISTOGRAM_BOOLEAN("Notifications.UsingNativeNotificationCenter",
                           success);
   }
+#endif
 
   if (!success) {
     // Fall back to the message center if initialization failed. Initialization