Reland: service worker: Add WPT tests for resultingClientId.

Reland of r592363, which was reverted due to timeouts. The timeout issue
should be fixed by r594723, which changed the mechanism of sending the
"last url" message from the navigated frame.

Other revisions from the original patch:
- Removed the unused expected_tags parameter from the default
  variant test function.
- Removed the dummy message resolver for "last url" from
  client_variant_test, which is no longer needed after the change
  mentioned above.
- Fixed a bug in redirect-worker.js's getClients(), where
  we returned instead of continued to the next iteration.

Original commit description:
> This adds a ?client variant to navigation-redirect.https.html, which
> does a lot of redirect tests. This tests the following:
> - resultingClientId is the id of the client, if one was created via
>   that request.
> - get(resultingClientId) for requests that did not create a client
>   resolves with undefined.
> - resultingClientId is reset on cross-origin redirects.
> - Client.url is the creation URL.
>
> Bug: 778497
> Change-Id: I08387e96a97df4656d800637862b67b7c8466e14
> Reviewed-on: https://chromium-review.googlesource.com/1206054
> Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
> Commit-Queue: Matt Falkenhagen <falken@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#592363}

Bug: 778497
Change-Id: I770f98cef7618c89c1b54245bc89a9c53a19bd5c
Reviewed-on: https://chromium-review.googlesource.com/1250306
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595249}