Improve off-thread cancellation performance [M1329901 and deps]

[classilla]

Land first:
https://hg.mozilla.org/releases/mozilla-beta/rev/554bbde4e462
Then
https://hg.mozilla.org/mozilla-central/rev/c98510adc780
https://hg.mozilla.org/releases/mozilla-aurora/rev/94debbbf7e5a
https://hg.mozilla.org/mozilla-central/rev/3df85eb27d47