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This is to run testharness.js as module scripts (e.g. via `import`).

For the same purpose, this CL also fixes undeclared variable
in Chromium's testharnessreport.js.

Bug: https://github.com/web-platform-tests/wpt/issues/12480
Change-Id: I1ff539676b754e27c9f5d50a355f5ca2fb9c07b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341230Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796696}