ServiceWorker: Remove [Unforgeable] idl extended attribute.

Remove [Unforgeable] idl extended attribute in ServiceWorker and CacheStorage.

Related spec issue:
  https://github.com/slightlyoff/ServiceWorker/issues/897

BUG=627777

Review-Url: https://codereview.chromium.org/2102093002
Cr-Commit-Position: refs/heads/master@{#406020}
14 files changed