Use tighter typing in editing: IndentOutdentCommand / InsertListCommand

Use tighter typing in editing to increase code readability and avoid unnecessary
type checks. This CL focuses on IndentOutdentCommand and InsertListCommand.

R=leviw@chromium.org, yosin@chromium.org

Review URL: https://codereview.chromium.org/421793002

git-svn-id: svn://svn.chromium.org/blink/trunk@179195 bbb929c8-8fbe-4397-9dbb-9b2b20218538