Upstream service worker `fetch` tests to WPT

The Web Platform Tests project includes equivalent versions of the
following tests:

- third_party/WebKit/LayoutTests/http/tests/serviceworker/fetch-canvas-tainting.html
- third_party/WebKit/LayoutTests/http/tests/serviceworker/fetch-cors-xhr.html

Remove the above Chromium-specific files to promote the use of the more
widely shared versions.

The following test file is more thorough than the equivalent version in
WPT. It contains the same assertions as those found in the WPT version,
but it also includes versions with an alternative value of the `img-src`
CSP directive.

- third_party/WebKit/LayoutTests/http/tests/serviceworker/fetch-csp.html

Extend the WPT version with the new assertions and remove the
Chromium-specific file. Also remove infrastructure configuration which
previously prevented the WPT version from being executed.

BUG=688116, 658997
R=falken@chromium.org

Review-Url: https://codereview.chromium.org/2805313003
Cr-Commit-Position: refs/heads/master@{#466697}
9 files changed