Add ServiceWorkerContextClient::getClients() interface

To implement ServiceWorkerGlobalScope.clients:
https://github.com/slightlyoff/ServiceWorker/blob/master/service_worker.ts#L205

BUG=363965

Review URL: https://codereview.chromium.org/240253002

git-svn-id: svn://svn.chromium.org/blink/trunk@172023 bbb929c8-8fbe-4397-9dbb-9b2b20218538