Service Worker: Add Clients.get(id)

As per the latest spec update, this CL implements Clients.get(id) method.

Spec discussion: https://github.com/slightlyoff/ServiceWorker/issues/723#issuecomment-151326898
Spec: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#clients-get-method

Intent to implement and ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/XlBHOIOCjUk

The layout tests for this CL are mostly taken from Mozilla's work instead of
making new ones to ensure compatibility: https://bugzilla.mozilla.org/show_bug.cgi?id=1222464

BUG=552310

Review URL: https://codereview.chromium.org/1439333002

Cr-Commit-Position: refs/heads/master@{#376870}
27 files changed