Ensure JavaScript equality when serviceWorker.register resolves to an existing worker [3/3]

Three-sided patch:
[1] Blink: https://codereview.chromium.org/409403002
[2] Chromium: https://codereview.chromium.org/412833002/
[3] Blink: test (THIS PATCH)

BUG=395928

Review URL: https://codereview.chromium.org/411863003

git-svn-id: svn://svn.chromium.org/blink/trunk@178938 bbb929c8-8fbe-4397-9dbb-9b2b20218538