Have HTMLConstructionSite::createHTMLElement() return an HTMLElement

Have HTMLConstructionSite::createHTMLElement() return an HTMLElement instead
of an Element for clarity.

R=eseidel@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/425263006

git-svn-id: svn://svn.chromium.org/blink/trunk@179250 bbb929c8-8fbe-4397-9dbb-9b2b20218538