Bug 841801 - Enable finalization of objects on the background thread r=billm

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extra : rebase_source : a08fb0b68aeffac400d300715ac874cfcbf535fc
Bug 841801 - Sweep some objects in the background r=billm

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extra : rebase_source : 8940bfbb1f223762bbc5ef38827467aa76505af1
Bug 841801 - Keep finalizable objects out of the nursery r=billm

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extra : rebase_source : ea13c882c82554a1510df47aadbc1bb1c6b6e569
Bug 841801 - Allow proxies to be finalized in the background r=billm

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extra : rebase_source : 48da96daddf75f505bf90c8e2ce2cdd6daf04cc5