Worklets are only able to communicate with the callback that they're supposed to call, or by console.log. In writing a test for https://codereview.chromium.org/2452073002 , I couldn't figure out a way to write a testharness-based test that passed the presubmit. Seems like console.log would be the way to give output, but presubmit complains:

* The following files are passing testharness results without console error messages, they should be removed:
 /b/build/slave/linux/build/src/third_party/WebKit/LayoutTests/http/tests/worklet/immutable-prototype-paint-worklet-expected.txt
ERROR: found passing testharness results without console error messages.

Other paint worklet tests are based on testrunner, which seems to go around this. However, my understanding is that new tests should be written with testharness. What do you recommend for writing tests for paintworklet?