Use tighter typing in editing: FormatBlockCommand / FrameSelection

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

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/420103002

git-svn-id: svn://svn.chromium.org/blink/trunk@179170 bbb929c8-8fbe-4397-9dbb-9b2b20218538