Add backspace-merge-into-list-item.html

This file was committed in https://crrev.com/bb59e5b729913a2023ea70e50d0689626565dd7e
However, it seems to be accidentally removed or not be committed so this CL
adds the file again.

BUG=none

Review-Url: https://codereview.chromium.org/2209493002
Cr-Commit-Position: refs/heads/master@{#409416}
1 file changed