Bug 1067699 (part 1) - Export double-conversion.h normally from MFBT. r=froydnj.

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extra : rebase_source : 14c3d827f4d8015d8326612e82782c52dcf7335d
Bug 1067699 (part 2) - Add mfbt/JSONWriter.h and use it for memory reporting. r=froydnj.

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extra : rebase_source : 7eb21c4c20c0434b3b236d1c53cbef9cb6a4759c