Backed out changeset 4088133a80e6 (bug 940779). It was the wrong fix.

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extra : rebase_source : 91ba320384e6ad196d0f8ce923e7bc33de45388a
Bug 940779 - MessageEventInit is a dictionary, r=bent

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extra : rebase_source : 37d006cf615cc6e7ed2a7141df54d9b948248946