Data only blueprints are not not nativized when flagged to do so

Data only blueprints that are explicitly flagged for nativization do not nativize. (Makes sense that they don't when they're not explicitly told, but if someone goes out of the way to flag them to, we probably should.)

Found on //UE4/Release-4.15 CL-3269047

1. Open UE4Editor.exe QAGAME
2. Open Project Settings -> Packaging
   1. Set Blueprint Nativization Method to Exclusive
   2. Set Use Pak File to false/unchecked (so we can see the assets)
3. Create a Child blueprint of QA_TestHelper, open it.
4. Select 'Class Settings' in the blueprints tool bar
5. Check 'Nativize' in the class settings.
6. Compile and save the child blueprint.
7. Compile and save QA_TestHelper (should be dirtied by last step)
8. File > Package > Win64
9. Once packaged, open the resulting directory ../QAGame/Content/Blueprints

Result: Even though we flagged the child, data-only, blueprint for nativization, it was not nativized and is still present in the blueprints folder.

Expected: Child blueprint asset is not in the blueprint folder, as it's been nativized.