DevTools: Get rid of addStyleClass and removeStyleClass on Element.prototype

Use DOMTokenList (element.classList) methods instead.

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

Review URL: https://codereview.chromium.org/102003005

git-svn-id: svn://svn.chromium.org/blink/trunk@163538 bbb929c8-8fbe-4397-9dbb-9b2b20218538