DevTools: Move DOMExtension.js into the "ui" module

R=aandrey@chromium.org, pfeldman@chromium.org, eustas, pfeldman

Review URL: https://codereview.chromium.org/258623003

git-svn-id: svn://svn.chromium.org/blink/trunk@172516 bbb929c8-8fbe-4397-9dbb-9b2b20218538