[ServiceWorker] Use the cookie which is differnt from the username in LayoutTest.

Currently we use the cookie name which is same as the username.
But I will add tests which change the username.
For the preparation of them, this patch changes LayoutTest to use the cookie which is differnt from the username.

BUG=N/A
TEST=http/tests/serviceworker

Review URL: https://codereview.chromium.org/722603002

git-svn-id: svn://svn.chromium.org/blink/trunk@185265 bbb929c8-8fbe-4397-9dbb-9b2b20218538