Import wpt@555b4870f99e18f149b45753b1c17862eace66ab

https://github.com/web-platform-tests/wpt/compare/27862155c...555b4870f

Using wpt-import in Chromium 5d6332967e809888701d745f502a362237f0c22e.
With Chromium commits locally applied on WPT:
  c178061101 "[LCP WPT] Fix test expectation in background-image- [...]
  d2dc655803 "WPT: fix overuse of PREFETCH_PROXY_BYPASS_HOST"

Note to gardeners: This CL imports external tests and adds expectations
for those tests; if this CL is large and causes a few new failures,
please fix the failures by adding new lines to TestExpectations rather
than reverting. See:
https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md

Directory owners for changes in this CL:
hbos@chromium.org, hta@chromium.org, guidou@chromium.org, phancke@microsoft.com:
  external/wpt/webrtc

NOAUTOREVERT=true
R=rubber-stamper@appspot.gserviceaccount.com

No-Export: true
Validate-Test-Flakiness: skip
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: I738a7713c1cec7d8b123704c5a719b20822dabc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6653061
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: WPT Autoroller <wpt-autoroller@chops-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1476525}