Enable LayoutNG for web_tests

Second attempt to enable LayoutNG for web_tests. Change one of many as
it isn't feasible to update all platforms & test expectations at once.

Bug: 966166
Tbr: wangxianzhu@chromium.org
Change-Id: Icc49f6d733d2a78cae5636c44e5605fca6fd6f24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651763
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669542}