Each LayoutTest shouldn't register SW for root/top-level scope

Currently registering something for a global scope could make
other tests fail due to issue 370773, let's always use
narrower scope and unregister the scope before the test runs.

(We'll need to have longest-prefix-match tests separately when
we fix the bug)

BUG=370777
TEST=http/tests/serviceworker/*

Review URL: https://codereview.chromium.org/270613003

git-svn-id: svn://svn.chromium.org/blink/trunk@173537 bbb929c8-8fbe-4397-9dbb-9b2b20218538