ServiceWorker: Add test for navigator.serviceWorker.installing


BUG=379012
TEST=run_webkit_tests.py http/tests/serviceworker/installing.html

Review URL: https://codereview.chromium.org/372453002

git-svn-id: svn://svn.chromium.org/blink/trunk@177720 bbb929c8-8fbe-4397-9dbb-9b2b20218538