Bug 909534 - don't call exit as a backstop in AsmJSSignalHandler (r=bhackett)

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extra : rebase_source : 12f00124b6c1806e0628d3eda0a47e474b24327d
Bug 909534 - tidy up AsmJSSignalHandler, remove unnecessary SIGBUS handling (r=bhackett)

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extra : rebase_source : a02fa6e374ea610db1e964181b84e251e5e44cbf
Bug 904826 - Remove some unneeded nsLayoutStatics calls. r=smaug,bholley,bz