[LayoutNG] Fix list-item marker logic

Update the list-item marker logic to reflect that list-style-image takes
precedence over list-style-type: none. Also adds a WPT correctness test.

Bug: 976937
Test: wpt/css/css-lists/list-type-none-style-image.html
Change-Id: I2255254f72e58d53671337ccb3f6a285cc515522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671896
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671772}
3 files changed