Add ENABLE_NATIVE_NOTIFICATIONS guard for fixing link errors

When the build flag enable_native_notifications = false, link errors
happen. This CL adds ENABLE_NATIVE_NOTIFICATIONS guard to fix
this problem.

Bug: None
Change-Id: I5bc5a3770a7d15b2f2f9de32c92722ec4fad0946
Reviewed-on: https://chromium-review.googlesource.com/1014182Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Joone Hur <joone.hur@intel.com>
Cr-Commit-Position: refs/heads/master@{#551112}

chrome/browser/notifications/notification_display_service_impl.cc

@@ -211,6 +211,7 @@ void NotificationDisplayServiceImpl::Display(
     return;
   }
 
+#if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
   NotificationPlatformBridge* bridge =
       NotificationPlatformBridge::CanHandleType(notification_type)
           ? bridge_
@@ -219,6 +220,7 @@ void NotificationDisplayServiceImpl::Display(
 
   bridge->Display(notification_type, profile_, notification,
                   std::move(metadata));
+#endif
 
   NotificationHandler* handler = GetNotificationHandler(notification_type);
   if (handler)
@@ -235,6 +237,7 @@ void NotificationDisplayServiceImpl::Close(
     return;
   }
 
+#if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
   NotificationPlatformBridge* bridge =
       NotificationPlatformBridge::CanHandleType(notification_type)
           ? bridge_
@@ -242,6 +245,7 @@ void NotificationDisplayServiceImpl::Close(
   DCHECK(bridge);
 
   bridge->Close(profile_, notification_id);
+#endif
 }
 
 void NotificationDisplayServiceImpl::GetDisplayed(