Bug 839376 (part 1) - Add an exact root for jstypedarray.cpp.  r=sfink.

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extra : rebase_source : fb5dd6bbd53ac0949070f574a636ec480131741b
Bug 839376 (part 2) - Add some easy exact roots for jsinfer.cpp.  r=sfink.

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extra : rebase_source : 38732b05549e2f322b33ce4728ae2e38f5b82e56
Bug 839376 (part 3) - Some low-hanging exact rooting fruit.  sfink.

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extra : rebase_source : 1d236895915f9474fd8e50c399bf2cd8139a6b5f