Convert fast/overflow/scrollbar-click-retains-focus.html with testharness

This patch converts the layout test with W3C testharness to reduce usage
of deprecated js-test.js.

This patch is also a preparation for using Element.innerText for text
dump. As the layout tests adds float style to all <span>, without this
patch, innerText dump can be confusing.

Bug: 887148
Change-Id: Ia8cafbc996fe1f4ae66350b565b7611bb7ec35e5
Reviewed-on: https://chromium-review.googlesource.com/c/1318357
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605574}