Intermittent FAIL in `/fetch/fetch-later/send-on-deactivate.https.window.html`

[TimvdLippe]

  ▶ TIMEOUT /fetch/fetch-later/send-on-deactivate.https.window.html
  │   ▶ FAIL [expected TIMEOUT] fetchLater() sends on navigating away a page w/o BFCache.
  │   │   → assert_equals: expected (object) object "[object Object]" but got (undefined) undefined
  │   │ 
  │   │ @https://web-platform.test:8443/fetch/fetch-later/send-on-deactivate.https.window.js:106:16
  │   │ async*parallelPromiseTest/<@https://web-platform.test:8443/fetch/fetch-later/resources/fetch-later-helper.js:11:21
  │   │ Test.prototype.step@https://web-platform.test:8443/resources/testharness.js:2869:25
  │   │ async_test@https://web-platform.test:8443/resources/testharness.js:681:34
  │   │ parallelPromiseTest@https://web-platform.test:8443/fetch/fetch-later/resources/fetch-later-helper.js:10:13
  │   └ @https://web-platform.test:8443/fetch/fetch-later/send-on-deactivate.https.window.js:83:20
  │   ▶ FAIL [expected TIMEOUT] fetchLater() with activateAfter=1m sends on page entering BFCache if BackgroundSync is off.
  │   │   → assert_equals: Number of sent beacons does not match expected count: expected 1 but got 0
  │   │ 
  │   │ expectBeacon@https://web-platform.test:8443/fetch/fetch-later/resources/fetch-later-helper.js:165:18
  │   │ async*@https://web-platform.test:8443/fetch/fetch-later/send-on-deactivate.https.window.js:182:9
  │   │ async*parallelPromiseTest/<@https://web-platform.test:8443/fetch/fetch-later/resources/fetch-later-helper.js:11:21
  │   │ Test.prototype.step@https://web-platform.test:8443/resources/testharness.js:2869:25
  │   │ async_test@https://web-platform.test:8443/resources/testharness.js:681:34
  │   │ parallelPromiseTest@https://web-platform.test:8443/fetch/fetch-later/resources/fetch-later-helper.js:10:13
  └   └ @https://web-platform.test:8443/fetch/fetch-later/send-on-deactivate.https.window.js:148:20

https://github.com/servo/servo/actions/runs/18176682818/job/51744901887

Should be fixed with #39545