Bug 1611498 [wpt PR 21416] - Fix scroll to text WPT flakiness, a=testonly

Bug 1611498 [wpt PR 21416] - Fix scroll to text WPT flakiness, a=testonly

Automatic update from web-platform-tests
Fix scroll to text WPT flakiness

Since stash.py polls until results are available, the tests can time out
in a single threaded environment if the test page tries to fetch results
before the target page stashes them. This patch moves the polling to
stash.js instead, so that the target page is free to stash results.

Also adds a window.close() call on the target page after stashing
results, otherwise each test case leaves a leftover window which can
slow down the test environment.

Bug: https://github.com/web-platform-tests/wpt/issues/20269
Change-Id: Ia2df8f889596e91207aca92f6ecb290c9f96d201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2018099
Commit-Queue: Nick Burris <nburris@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735578}

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wpt-commits: 9912dbe9d6cb67fdfa3f86b662c1c205d5fd6242
wpt-pr: 21416