Bug 999140 - Mapped array buffers need a safety buffer, r=Waldo

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extra : rebase_source : 03e97115af90ea27f6f2a658bea5af6552b2a588
Bug 999140 - Implement createMappedArrayBuffer for testing, r=Waldo

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extra : rebase_source : f9304f99349cbec76dcb50f67353dcd4e95e506e