Limit Link header based SW installations for fetches made by SW.

To prevent a service worker from spawning new service workers in the
background, only process link headers on requests made by service
workers if the service worker is controlling at least one client.

BUG=662443

Review-Url: https://codereview.chromium.org/2512103003
Cr-Commit-Position: refs/heads/master@{#433489}
3 files changed