Upstream service worker opaque-response-preloaded.https.html test to WPT

- Re-locate file for eventual submission to the Web Platform Tests
  project
- Add "clean up" logic to remove iframe from document
- Remove implementation-specific details from in-line documentation

BUG=688116, 625575
R=falken@chromium.org

Review-Url: https://codereview.chromium.org/2877673004
Cr-Commit-Position: refs/heads/master@{#471341}
5 files changed