This patch:
- changes Puppeteer-Firefox plumbing of defaultBrowserOptions to align
with the way we do it for Puppeteer.
- plumbs puppeeteer-dependent Errors and DeviceDescriptors down to every
test.
- unifies a few tests between Puppeteer-Firefox and Puppeteer.
**Note:** in future, we should expose errors as `puppeteer.errors` and
device descriptors as `puppeteer.devices` to make it easy to pass around
Puppeteer/Puppeteer-Firefox instance.
References #3889.
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.