Use tighter typing in editing: htmlediting create methods

Use tighter typing in editing/ to increase code readability.
This CL focuses on the create*Element() functions in htmlediting.

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

Review URL: https://codereview.chromium.org/422693002

git-svn-id: svn://svn.chromium.org/blink/trunk@179173 bbb929c8-8fbe-4397-9dbb-9b2b20218538