Use tighter typing in editing: InsertParagraphSeparatorCommand / InsertTextCommand

Use tighter typing in editing/ to increase code readability. This CL focuses on
InsertParagraphSeparatorCommand / InsertTextCommand.

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

Review URL: https://codereview.chromium.org/423003003

git-svn-id: svn://svn.chromium.org/blink/trunk@179180 bbb929c8-8fbe-4397-9dbb-9b2b20218538