ServiceWorker: Rename Client interface to ServiceWorkerClient

The interface was given a less generic name in a recent spec update:
https://github.com/slightlyoff/ServiceWorker/issues/350

BUG=403536
R=falken@chromium.org,haraken@chromium.org

Review URL: https://codereview.chromium.org/463423003

git-svn-id: svn://svn.chromium.org/blink/trunk@180291 bbb929c8-8fbe-4397-9dbb-9b2b20218538