DevTools: Reduce code via using document.createElementWithClass and document.createChild.

R=vsevik, apavlov

Review URL: https://codereview.chromium.org/362273002

git-svn-id: svn://svn.chromium.org/blink/trunk@177390 bbb929c8-8fbe-4397-9dbb-9b2b20218538