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We re-land the initial change so that we can support importing and running
tests from the w3c without needing to generate and maintain -expected.txt
baselines for them (though we do support such baselines).

The change from the prior version is to add a header and a footer to the
output in testharnessreport.js, so that we can reliably detect that this
is a testharness-based test (and that we've outputted all the assertions we
have).

R=ojan@chromium.org
BUG=268711

Review URL: https://codereview.chromium.org/109763007

git-svn-id: svn://svn.chromium.org/blink/trunk@163996 bbb929c8-8fbe-4397-9dbb-9b2b20218538