Validate WebCodecs encoder's flush promise handling
Previously, the WebCodecs encoder modified how it handles promises
associated with flush requests in Bug 1892785 [1]. This patch introduces
tests to ensure that the encoder's handler for a flush request can
correctly resolve or reject the associated promise without affecting
other pending promises.
[1] https://hg.mozilla.org/mozilla-central/rev/eeac4e97050a4b1e61d8331f5d61b5b3316d4f72
Depends on D207655
Differential Revision: https://phabricator.services.mozilla.com/D208689
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1891897
gecko-commit: ae143f29809b0546d228d52c340b3ba476db0020
gecko-reviewers: media-playback-reviewers, padenot