Bug 982726 - Patch 1 - Service Worker: ServiceWorkerClients and Client interfaces with getServiced(). r=baku

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extra : rebase_source : f8dd649fd30cfffb2a361aa24868e83413e7b54c
Bug 982726 - Patch 2 - Add postMessage to service worker. r=baku

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extra : rebase_source : 55474dcece43eb53a99829ec0a076f2f1e09ce22
extra : amend_source : 39daf21dd3187542754f82115d75eaca20b838b5
Bug 982726 - Patch 3 - Implement client.postMessage and add tests for getServiced(). r=baku

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extra : rebase_source : 0062dc40cd5cb1248bdce4c70b8abf389eed073d
extra : amend_source : 615bc61390c7e323ddca5a16352387bd4a7c920a
Bug 982726 - Patch 4 - Extensive testing of postMessage. r=baku

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extra : rebase_source : c2a5b3e61cd314c995cc1935aa928247301cb9c5
extra : amend_source : a970a4e351b2b7064fbccbed6b74dedec609f878