Move Node::enclosingBlockFlowElement() to htmlediting.h

Move Node::enclosingBlockFlowElement() / inSameContainingBlockFlowElement() to
htmlediting.h as this is only needed for editing.

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

Review URL: https://codereview.chromium.org/421293004

git-svn-id: svn://svn.chromium.org/blink/trunk@179081 bbb929c8-8fbe-4397-9dbb-9b2b20218538