Bug 942255 - UsesSystemPrincipal() in Promise::EnabledForScope() on Workers. r=bz

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extra : rebase_source : 3a1351a488ed2f90c6e998cf396639d0bb0d267c
Bug 942256 - ThreadSafeCheckIsChrome() checks UsesSystemPrincipal() for Workers. r=bz

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extra : rebase_source : 756edcea9860c22b06820ef505f51e7741cd5442