Bug 969415 (part 1) - Add OOM callback hook to JS engine. r=njn.

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extra : rebase_source : f6a7f0b0aa10d6a938e1608f6cb7ccf76f3085c8
Bug 969415 (part 2) - Add a pref to enable memory report dumping on JS OOMs. r=luke.

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extra : rebase_source : 3971f809744869a2f2d77e435e95aca44bf7e9d7