Insert placeholder when removing selection at end of editable after line break

This patch changes delete selection command to insert a placeholder when deleting selection at end of editable root after line break.

This patch fixes regression introduced by r154347, Avoid adding placeholder when deleting last text in root.

BUG=264986
TEST=LayoutTests/editing/deleting/delete-character-002.html, LayoutTests/editing/deleting/delete-character-003.html
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/21044002

git-svn-id: svn://svn.chromium.org/blink/trunk@156139 bbb929c8-8fbe-4397-9dbb-9b2b20218538