This changes the call to base.test.util.UrlUtils to a native OS
method. We may need to update this when Chromium starts running
tests on "R". At that point we'll need a non-base mechanism to
pass the right folder to ANGLE. This could be via env vars,
debug properties, command-line arguments, or #defines.

The prior implementation is left as commented-out code as a
reminder to fix later.

Also updates WARN() to std::cerr because WARN() was not showing
up when testing.

Bug: chromium:1097957
Change-Id: I4a84ea007341dbe7fe2184eac3aae0ddca44cc9c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2818240
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>