Fix the Windows non-threadsafe build by exposing/using a thread-aware symbol only in threadsafe builds.  No bug, r=terrence

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extra : rebase_source : 5f950f54bad34f2fd93da575aff332f155a684aa
Bug 948227 - Add code to warn when the __proto__ setter is called that it's very slow and shouldn't be used.  (Don't use it yet, tho, because it requires the second part of bug 948583 to land first.  Once that lands, fully enabling this just requires some uncommenting.)  r=efaust

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extra : rebase_source : 87d320edcc99b6ef7df98c2470c78d4085aa4f99