Make ServiceWorker::from handle "unwrapping" a null ServiceWorker
Make ServiceWorker::from handle "unwrapping" a null ServiceWorker

Currently ServiceWorkers are non-null, however when we implement the .waiting property, the browser will "set" a null ServiceWorker to clear the property. Make Blink ready to unwrap one of those.

BUG=379012

Review URL: https://codereview.chromium.org/323523002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@175853 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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