Use tighter typing in editing: DeleteSelectionCommand

Use tighter typing in editing to increase code readability.
This CL focuses on DeleteSelectionCommand.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/420053002

git-svn-id: svn://svn.chromium.org/blink/trunk@179072 bbb929c8-8fbe-4397-9dbb-9b2b20218538