Upstream service worker `fetch` tests to WPT

**fetch-request-css-base-url**

The Chromium-specific version of this test is almost identical to the
version provided by the Web Platform Test suite. The upstream version
differs only in its use of Promises for control flow. However, the
upstream version also suffers from timeout issues in the build
environment.

Rename the Chromium-specific version to reflect its deprecated status,
and re-enable the upstream version as an expected timeout.

**fetch-request-xhr**

Update the upstream version to improve failure reporting and to include
novel tests from the Chromium version for HTTP header values (modified
to comply with the Fetch specification).

Update the Chromium-specific version to document its deprecated status
and invalid assertions.

Update both versions to improve state management via `Test#add_cleanup`.

**fetch-response-xhr**

Update the upstream version to avoid a race condition.

Update the Chromium-specific version to document its deprecated status.

Update both versions to improve state management via `Test#add_cleanup`.

BUG=688116, 595993, 658997
R=mek@chromium.org

Review-Url: https://codereview.chromium.org/2862353002
Cr-Commit-Position: refs/heads/master@{#470395}
10 files changed