ServiceWorker: Fix ServiceWorkerRegistration object handling

When there is an existing ServiceWorkerRegistration, getOrCreate() shouldn't
release the given WebServiceWorkerRegistration object because it has already
been taken by the existing registration.


BUG=396400
TEST=https://codereview.chromium.org/466723002/

Review URL: https://codereview.chromium.org/469273002

git-svn-id: svn://svn.chromium.org/blink/trunk@180262 bbb929c8-8fbe-4397-9dbb-9b2b20218538