[ServiceWorker] Fix bugs of wpt tests for Client.navigate

This CL
 - modifies is_service_worker() in external/wpt/resources/testharness.js
   so that frame.contentWindow.navigator.serviceWorker.controller CAN be
   identified correctly as ServiceWorker object within main frame's
   execution context, this ensures success of fetch_tests_from_worker,
   which is called by several tests in client-navigate.https.html.
 - throws TypeError if navigation failed for Client.navigate,
   E.g. mixed-content violation.

BUG=658997
TEST=blink_tests
external/wpt/service-workers/service-worker/client-navigate.https.html

Review-Url: https://codereview.chromium.org/2912593003
Cr-Commit-Position: refs/heads/master@{#478561}
6 files changed