Bug 1689115 [wpt PR 27351] - [testharness.js] Disable assert tracking on workers...

Bug 1689115 [wpt PR 27351] - [testharness.js] Disable assert tracking on workers, a=testonly

Automatic update from web-platform-tests
[testharness.js] Disable assert tracking on workers (#27351)

We are continuing to see a performance regression from assert tracking
in Chromium. After investigation, it appears that the root problem is
that there is no way currently for a testharness.js script loaded in a
worker to know that it shouldn't be tracking asserts (since
testharnessreport.js isn't loaded).

To fix the performance hit, this commit disables assert tracking
entirely in workers. This is a behavior regression for the feature, but
is necessary to avoid an unacceptable performance regression.

Associated Chromium bug: https://crbug.com/1170588
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wpt-commits: a1f5466c51ef000b4b4fcf66473e4f9a2b11c503
wpt-pr: 27351