service worker: When claiming, don't assume document_url is valid

      Many provider hosts have an empty document_url, for example those
that haven't yet been loaded or those created for special URLs like
chrome-search://. So that claim can use IsContextSecureForServiceWorker,
return false when the URL is invalid instead of doing a
DCHECK that it's valid.

BUG=621762,607543

Review-Url: https://codereview.chromium.org/2085923002
Cr-Commit-Position: refs/heads/master@{#401216}