[css-flexbox] Use an existing file for a blank reference

about:blank doesn't work, at least in the Chromium test harness,
because it concatenates it to a path and then doesn't find the file.

Bug: 963168
Change-Id: I377ee38ad1df1039e89257673b71c983099bab55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629332
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663351}
2 files changed