Remove ContextLifecycleObserver from NavigatorServiceWorker

NavigatorServiceWorker needs to reset m_serviceWorker when the context
gets destroyed. This CL makes ServiceWorkerContainer do the reset work
and removes ContextLifecycleObserver from NavigatorServiceWorker.

See the discussion on the CL for more rationale.

BUG=610176

Review-Url: https://codereview.chromium.org/2556893003
Cr-Commit-Position: refs/heads/master@{#438475}
5 files changed