Instead of requiring all frames in the subtree to receive a particular
event, we rely on the browser's definition of load and DOMContentLoaded.
This changes logic in a few edge cases:
- Some browsers do not emit load event upon window.stop() at all.
- DOMContentLoaded does not wait for subframes, so they might not be
ready when passing `{ waitUntil: 'domcontentloaded' }`.
`networkidle` preserves the old logic.
