Define platformInfo for NaCl extensions in the CWS

The extension was unable to download an extension with a NaCl module,
because the CWS/CRX url was missing some platform information.
This missing info lead to a "204 No Content" response.

See
- https://code.google.com/p/chromium/issues/detail?id=235233
- https://code.google.com/p/chromium/issues/detail?id=180402