These getters are introduced as a more convenient substitute for
a `require('puppeteer/Errors')` and
`require('puppeteer/DeviceDescriptors')`.
This way we can make cross-browser story nicer - a single require
of `puppeteer` or `puppeteer-firefox` fully defines Puppeteer
environment.
This patch refactors Puppeteer-Firefox code to declare public
API in `/lib/api.js` and use it to setup async stack hooks
over the public API method calls.
