ServiceWorker: Use FrozenArray for *MessageEvent.ports.

The T[] array type has ben removed from WebIDL. Instead we should use
FrozenArray in ServiceWorkerMessageEvent and ExtendableMessageEvent.

Related spec issue:
  https://github.com/slightlyoff/ServiceWorker/issues/810

BUG=628911

Review-Url: https://codereview.chromium.org/2155943002
Cr-Commit-Position: refs/heads/master@{#406230}
2 files changed