Upstream service worker header test to WPT

- Re-locate file for eventual submission to the Web Platform Tests
  project
- Improve "clean up" logic to consistently remove iframe from document
- Remove unnecessary `<script>` tags

BUG=688116, 537127
R=falken@chromium.org

Review-Url: https://codereview.chromium.org/2878683002
Cr-Commit-Position: refs/heads/master@{#471338}
3 files changed