Use tighter typing in editing/

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

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/413363004

git-svn-id: svn://svn.chromium.org/blink/trunk@178950 bbb929c8-8fbe-4397-9dbb-9b2b20218538