Chrome canary and stable runs are intermittently hanging

Initially identified by @jgraham:

https://wpt.fyi/runs?label=master&label=stable&from=2025-12-08T00%3A00 looks like Chrome stable runs are hanging: https://github.com/web-platform-tests/wpt/runs/58035842405 Maybe also in nightly(!) https://github.com/web-platform-tests/wpt/runs/58035840099

Based on my initial investigation, it looks like the job shard named wpt-chrome-{canary|stable}-testharness-7 takes a significantly longer time to complete than the other shards. On a sucessful run, it tends to finish in around 170 minutes, which is 10 minutes away from the 180 minute timeout. At first glance, I am not seeing any specific tests in the logs that are taking substantially longer to complete. It could be that these failures are just a coincidental side effect of the run time getting too close to the timeout.

Firefox runs of this shard take a significantly shorter time to complete; around 60 to 90 minutes (example), so I am assuming there is something Chrome-specific about this increased run time.

Start with the linked GitHub Actions runs for wpt-chrome-{canary|stable}-testharness-7 and compare their logs with the successful Chrome and Firefox runs cited in the issue. Identify why the Chrome shard approaches or exceeds the 180-minute timeout; done means canary and stable runs complete reliably within that limit.