Uv_walk crash on web worker close

Editor assessment

The subject states symptom and trigger precisely: a uv_walk crash occurring on web worker close. The failing subsystem, the crash, and the lifecycle moment are all named, so a reader understands the defect without a diff. The fix type is right, no scope is attached, and the body's single bullet, switching to DCHECK_EQ, documents the concrete hardening without noise. No footers appear, and hygiene is clean. The message communicates the failure scenario and the remedy's character compactly, which fully satisfies the needs of a targeted crash fix.