Reland: service worker: Don't control a subframe of an insecure context

      We must check isSecureContext when creating the network provider to
adhere to https://w3c.github.io/webappsec/specs/powerfulfeatures/#settings-privileged.

We already did this for getRegistration(), register(), unregister() but must
also do this when deciding whether to control an in-scope document.

BUG=607543
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
TBR=reviewers from the original review

Original review: https://codereview.chromium.org/2009453002

Review-Url: https://codereview.chromium.org/2071433003
Cr-Commit-Position: refs/heads/master@{#400093}