Bug 1060398 - Fix ArrayPushDense to work with arrays with non-writable length. r=bhackett

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extra : rebase_source : 8ec8dcb49bca4516764107f5f246959cf9cc0442
Bug 1054545 - Make helperThreadCount() always return 0 in more-deterministic builds. r=bhackett

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extra : rebase_source : 253a4e0ad39334ef4c49e06628d893553e8ab031