Add a test, ServiceWorker statechange target is the installing worker.

It was not possible to write this assertion because == did not work
for ServiceWorker instances, which were created anew each time they
were marshalled to Blink. That works now.

BUG=361907

Review URL: https://codereview.chromium.org/311573002

git-svn-id: svn://svn.chromium.org/blink/trunk@175420 bbb929c8-8fbe-4397-9dbb-9b2b20218538