Move Node::isBlockFlowElement() to htmlediting.h

Move Node::isBlockFlowElement() to htmlediting.h as this is specific to
editing.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/416393005

git-svn-id: svn://svn.chromium.org/blink/trunk@179270 bbb929c8-8fbe-4397-9dbb-9b2b20218538