Use tighter typing in editing: BreakBlockquoteCommand & Caret

Use tighter typing in editing/ to increase code readability and avoid
unnecessary type checks. This CL focuses on BreakBlockquoteCommand and
Caret.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/419313002

git-svn-id: svn://svn.chromium.org/blink/trunk@178985 bbb929c8-8fbe-4397-9dbb-9b2b20218538