Bug 1027221 - Allow incremental GC barriers on off-main-thread permanent atoms; r=billm

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extra : rebase_source : 269fadc9dbb6dd9cffd6b87f01ff4b4fc4738ef4
Bug 1027221 - Test that promises can be resolved with a PermanentAtom string; r=terrence

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extra : rebase_source : 2b2bb9211b88b9ec491cf247cd8de055d41bf510