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

components/browser_sync/test_profile_sync_service.h

Index: components/browser_sync/test_profile_sync_service.h
diff --git a/components/browser_sync/test_profile_sync_service.h b/components/browser_sync/test_profile_sync_service.h
index c1f8dc27708ada10b210b85ea5cc8b047ada9964..59a5ca989ca379d9a8305465e238b2be799c3bcd 100644
--- a/components/browser_sync/test_profile_sync_service.h
+++ b/components/browser_sync/test_profile_sync_service.h
@@ -38,7 +38,7 @@ class TestProfileSyncService : public ProfileSyncService {
   syncer::SyncPrefs* sync_prefs() { return &sync_prefs_; }
 
  protected:
-  // Return NULL handle to use in backend initialization to avoid receiving
+  // Return null handle to use in backend initialization to avoid receiving
   // js messages on UI loop when it's being destroyed, which are not deleted
   // and cause memory leak in test.
   syncer::WeakHandle<syncer::JsEventHandler> GetJsEventHandler() override;