Limit foreign fetch to only intercept requests made from secure contexts.

Implements step 5 of "Handle Foreign Fetch" in [1]

[1]: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#on-foreign-fetch-request-algorithm

BUG=540509

Committed: https://crrev.com/dd933bda0baa6a13ab0120f0056a2b783e459efb
Review-Url: https://codereview.chromium.org/2032653003
Cr-Original-Commit-Position: refs/heads/master@{#398752}
Cr-Commit-Position: refs/heads/master@{#399870}
12 files changed