build freedom-for-firefox-for-uproxy and fix missing provider in freedom-for-chrome-for-uproxy

This builds `freedom-for-firefox-for-uproxy.jsm` and fixes an issue with `freedom-for-chrome-for-uproxy.js` in which the Chrome providers weren't included.

Tested by building `uproxy-networking` and symlinking `freedom-for-xxx.js` to `freedom-for-xxx-for-uproxy.js` and connecting to the echo server on port 9998.
Better to use: `require.resolve('freedom/Gruntfile')` (see docs on require.resolve) as that is more robust than hard-wiring paths.