Change |#if defined| to |#if BUILDFLAG| in privet_notifications.cc

Per https://codereview.chromium.org/2460653003/ ENABLE_MDSN has been migrated
from a #define to a buildflag; however, there are two instances of it in
PrivetNotificationService that have not been changed yet.  This CL fixes those.

Bug: 863508
Change-Id: I273afb99ced733012ce58dcfa061d18faa8508de
Reviewed-on: https://chromium-review.googlesource.com/1136864Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lily Houghton <lilyhoughton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575060}

chrome/browser/printing/cloud_print/privet_notifications.cc

@@ -323,7 +323,7 @@ void PrivetNotificationService::Start() {
 }
 
 void PrivetNotificationService::OnNotificationsEnabledChanged() {
-#if defined(ENABLE_MDNS)
+#if BUILDFLAG(ENABLE_MDNS)
   if (IsForced()) {
     StartLister();
   } else if (*enable_privet_notification_member_) {
@@ -352,7 +352,7 @@ void PrivetNotificationService::OnNotificationsEnabledChanged() {
 
 void PrivetNotificationService::StartLister() {
   ReportPrivetUmaEvent(PRIVET_LISTER_STARTED);
-#if defined(ENABLE_MDNS)
+#if BUILDFLAG(ENABLE_MDNS)
   traffic_detector_ = nullptr;
 #endif  // ENABLE_MDNS
   service_discovery_client_ =