Bug 1011854 - Remove usage of 2nd WeakMap#get parameter from SessionStore; r=ttaubert

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extra : rebase_source : a63bf620acd02ff5e72a0dc6821423d25a5f34e9
Bug 1011856 - Remove usage of 2nd WeakMap#get parameter from MozKeyboard; r=xyuan

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extra : rebase_source : 9ab7d3e454a56c74380b608b32b262f80b3e2e6a