[wpt] Fix css-overflow/incremental-scroll.html timeout
The test times out in all non-content-shell browsers 0 because the
removal of class=reftest-wait is guarded by window.testRunner, a
content shell-only API.
Bug: None
Change-Id: Ic5c87a0e1da6683f8e8bbf3585349dd20da7663b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5867178
Reviewed-by: Philip Rogers pdr@chromium.org
Commit-Queue: Philip Rogers pdr@chromium.org
Auto-Submit: Jonathan Lee jonathanjlee@google.com
Cr-Commit-Position: refs/heads/main@{#1356145}