Remove style spans to follow the styles of list item

This CL removes style spans to follow the styles of list item when
the text of the pasted or merged element becomes a list item.

BUG=335955
TEST=editing/deleting/backspace-merge-into-list-item.html

Review-Url: https://codereview.chromium.org/2072093002
Cr-Commit-Position: refs/heads/master@{#401533}
5 files changed