[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/test/fake_server/bookmark_entity_builder.cc

Index: components/sync/test/fake_server/bookmark_entity_builder.cc
diff --git a/components/sync/test/fake_server/bookmark_entity_builder.cc b/components/sync/test/fake_server/bookmark_entity_builder.cc
index f4ea23de19fa7a05263615d8f8c4d05d7a7ccfef..806d1dfec7f3bb916e24e4a85cce6f87c3c5223b 100644
--- a/components/sync/test/fake_server/bookmark_entity_builder.cc
+++ b/components/sync/test/fake_server/bookmark_entity_builder.cc
@@ -47,7 +47,7 @@ void BookmarkEntityBuilder::SetParentId(const std::string& parent_id) {
 std::unique_ptr<FakeServerEntity> BookmarkEntityBuilder::BuildBookmark(
     const GURL& url) {
   if (!url.is_valid()) {
-    return base::WrapUnique<FakeServerEntity>(NULL);
+    return base::WrapUnique<FakeServerEntity>(nullptr);
   }
 
   sync_pb::EntitySpecifics entity_specifics = CreateBaseEntitySpecifics();