Decouple script URL from ServiceWorkerRegistration
Before this patch, ServiceWorkerRegistration had a constant
script URL.
The spec has a different model: .installing, .waiting, and .active
workers for the same registration can be from different script URLs.
This patch removes script_url from SWRegistration, in preparation
for implementing the spec steps for same-scope, new-script-url
registration.
BUG=398355
Review URL: https://codereview.chromium.org/
501453002
Cr-Commit-Position: refs/heads/master@{#291882}