[Sync] Replacing NULL with nullptr/null throughout sync code.

chrome/browser/sync/sync_error_notifier_ash.cc

Index: chrome/browser/sync/sync_error_notifier_ash.cc
diff --git a/chrome/browser/sync/sync_error_notifier_ash.cc b/chrome/browser/sync/sync_error_notifier_ash.cc
index 5af98ade0895dc5263bce245a5ca4ef1d150a5a1..2a67912929aaa443f73c93f2c8ed450d037c55b3 100644
--- a/chrome/browser/sync/sync_error_notifier_ash.cc
+++ b/chrome/browser/sync/sync_error_notifier_ash.cc
@@ -114,14 +114,14 @@ SyncErrorNotifier::~SyncErrorNotifier() {
 
 void SyncErrorNotifier::Shutdown() {
   error_controller_->RemoveObserver(this);
-  error_controller_ = NULL;
+  error_controller_ = nullptr;
 }
 
 void SyncErrorNotifier::OnErrorChanged() {
   NotificationUIManager* notification_ui_manager =
       g_browser_process->notification_ui_manager();
 
-  // notification_ui_manager() may return NULL when shutting down.
+  // notification_ui_manager() may return null when shutting down.
   if (!notification_ui_manager)
     return;