This test was added in https://chromium-review.googlesource.com/c/chromium/src/+/1316104 and makes the win7_chromium_rel_ng that run the webkit layout tests very flaky. See https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win7_chromium_rel_ng?limit=200

The error is caused by an INFO line being printed when on Windows before 8 (which makes sense since it is a Win7 builder):

04:49:21.304 5188 worker/1 http/tests/priorities/resource-load-priorities-service-worker.https.html output stderr lines:
04:49:21.304 5188   [6148:5400:1109/044920.276:INFO:media_foundation_video_encode_accelerator_win.cc(347)] Windows versions earlier than 8 are not supported.