Bug 939843: Unify FloatingPoint's code for Double and Float; r=waldo

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extra : rebase_source : 2f7956d497d534af8931939a38a29ad27695a3fc
Bug 939843: Tests for mfbt::Floats; r=waldo

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extra : rebase_source : a6515f7d4e82e020fd7686f14932855873078813
Bug 939843: Required changes in Spidermonkey; r=mjrosenb

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extra : rebase_source : e629b045dfe460348c339ef0e7d301b67c6799fd
Bug 939843: Required changes in the rest of the tree; r=froydnj

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extra : rebase_source : 5757eb2b73755c8da08112b91b025a8fa860d7e8