Add blink-side plumbing for ServiceWorkerGlobalScope.clients

Follow-on to kinuko's crrev.com/240253002
* IDL/stubs for ServiceWorkerClients
* Promise plumbing for getServiced()
* Mapping ID results to Client[]
* IDL/stubs for Client

BUG=363965
R=dglazkov@chromium.org, kinuko@chromium.org

Review URL: https://codereview.chromium.org/245863003

git-svn-id: svn://svn.chromium.org/blink/trunk@172170 bbb929c8-8fbe-4397-9dbb-9b2b20218538