[wpt] Migrate custom-elements/ coverage to headless shell
See 0 and 1 for background. The test history 2 and CQ flakiness
check suggest the tests consistently run as expected on the new runner
and embedder.
ElementInternals-role.html is marked as slow because each testdriver
call is now implemented as several WebDriver requests; collectively,
executing testdriver takes (~0.1s / call) * O(100 calls) = O(10s),
which easily exceeds the test timeout. (The timeout doesn't appear on
wpt.fyi because the results are generated from an official build,
which is heavily optimized.)
Bug: 40927590
Change-Id: I51107cc657fb859eb5bc682101c416f065dc2e3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6020549
Reviewed-by: Joey Arhar jarhar@chromium.org
Commit-Queue: Joey Arhar jarhar@chromium.org
Auto-Submit: Jonathan Lee jonathanjlee@google.com
Cr-Commit-Position: refs/heads/main@{#1383210}