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.
This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.
As things become more stable, we'll gradually move it out of
the experimental folder.
