Use tighter typing in Editing: VisibleUnits

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

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

Review URL: https://codereview.chromium.org/434773003

git-svn-id: svn://svn.chromium.org/blink/trunk@179555 bbb929c8-8fbe-4397-9dbb-9b2b20218538