- Enable StyleSheet to resolve/reject a promise from Replace()
- Implement the StyleSheet::Replace() method.
- Remove use of `alternate` in WPTs, because it will soon be gone.
- Ensure that WPT test cases which modify document's style clean up after themselves.
- Modify WPT @import test cases to handle unhandled cases on promises.
- Modify WPT test case for failed imports to correctly check `NetworkError` as per the spec, rather than `NotAllowedError`.

Differential Revision: https://phabricator.services.mozilla.com/D62524

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extra : moz-landing-system : lando