Check that Service Workers are registered from secure origins.

This is in keeping with the Chromium proposal to provide powerful new
web platform features to secure origins:

http://www.chromium.org/Home/chromium-security/prefer-secure-origins-for-powerful-new-features

This is also in keeping with the Service Worker spec, which recommends
Service Workers use HTTPS, with some exceptions:

https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#security-considerations

BUG=394213

Review URL: https://codereview.chromium.org/400903002

git-svn-id: svn://svn.chromium.org/blink/trunk@178728 bbb929c8-8fbe-4397-9dbb-9b2b20218538