Prevent a service worker from keeping itself alive by self postMessage.

This changes the timeout for message events when the message was sent from
a service worker to be the curent timeout of that service worker. So while
a service worker can still post message to itself, this won't allow it to
stay alive any longer than a single event would.

BUG=647943,648836

Review-Url: https://codereview.chromium.org/2506263005
Cr-Commit-Position: refs/heads/master@{#436393}
5 files changed