WPT: CSS: Add same-origin tests for loading error stylesheets.

This adds a test that a stylesheet that failed to load is considered
cross-origin. That is, accessing |styleSheet.rules| throws a
SecurityError.  This aligns with the specifications:
- cssRules checks the `origin-clean` flag:
  https://drafts.csswg.org/cssom/#dom-cssstylesheet-cssrules
- This is set to true iff CORS-same-origin:
  https://html.spec.whatwg.org/multipage/links.html#link-type-stylesheet
- CORS-same-origin is false for "error" responses:
  https://html.spec.whatwg.org/multipage/urls-and-fetching.html#cors-same-origin

Bug: 911974
Change-Id: I558e6d3dfa564b15284c393ffc35b80f54a9bc4e
Reviewed-on: https://chromium-review.googlesource.com/c/1371307
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615479}
3 files changed