Upstream service worker `fetch` test to WPT

- Re-write expected "credentials" values according to WPT's definition.
- Create a dedicated sub-test for each request. This ensures deterministic
  failure and allows much finer-grained error reporting.

BUG=688116
R=mek@chromium.org

Review-Url: https://codereview.chromium.org/2856863003
Cr-Commit-Position: refs/heads/master@{#469767}
4 files changed