Bug997547 correspondence of reviews

Getting there, now we have the flag we need to fill it with a value.

We have a configuration file in contacts, that is populated during build time, you can see how this is filled here:
https://github.com/mozilla-b2g/gaia/blob/master/build/import-config.js#L67

And then you can access the configuration variable via:
https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/contacts/js/fb/fb_init.js#L32
https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/contacts/js/fb/fb_init.js#L38