Bug 984048 - Add non-null installing worker check to FinishInstall. r=ehsan

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extra : rebase_source : bb459bc5bddca875099584446c65b2909f92b343
Bug 984048 - Temporarily disable serviceworker updatefound test. r=test-only

The updatefound test uses some bits that haven't landed yet, namely the ability to access running ServiceWorker instances using navigator.serviceWorker.controller.

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extra : rebase_source : dfacd3352d6d09c68ae1debcc0ce5f81127161e7