Bug 875050, EventListenerManager mTarget should be EventTarget, r=bz

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extra : rebase_source : 92caa79eb16fbfad7b8cc6a6037db31f47e2f056
Bug 875050 - Add EventListenerAdded/Removed to EventTarget, r=bz

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extra : rebase_source : c7aea2048abd1d74ae02c1cfae84602cdf3af8c3
Bug 875050, make WebSocket to use EventListenerAdded/Removed, r=bz

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extra : rebase_source : fa0995983c4a6c738bae2d88152db65e59cc1d72