Guard against overriding window's native FormData accidentally

[anhang]

This is to address an issue when using twilio-chat Electron. twilio-chat depends on isomorphic-form-data, which updates global (aka window) in Electron.

[DylanPiercey]

@alexindigo @anhang I'm not sure this is the best idea here. The node implementation of FormData is not able to be run in a browser as a polyfill. If we are getting to the index.js then that means that we've not properly resolved the browser file, which means the node implementation is being loaded. I think this issue is more of a configuration issue as it seems the electron setup is not resolving the browser field in the package.json.