Bug 837751 - GC: Root CloneFunctionAtCallsite and WriteStructuredClone r=sfink

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extra : rebase_source : f049147afc19d8c2412223f028aef7adee1aae00
Bug 838643 - GC: Finalize shapes on background thread r=billm

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extra : rebase_source : 378e23a78b86b544806dcf326a173c7b3c7c39eb