DevTools: simple selector to return only classname if nodename is div

Most of the layoting in HTML is done with "div" elements, thus selectors
like "div.my-class" doesn't make more sense then ".my-class". This patch
changes simpleSelector method to return ".my-class" instead of
"div.my-class".

BUG=364052

Review URL: https://codereview.chromium.org/244443004

git-svn-id: svn://svn.chromium.org/blink/trunk@172033 bbb929c8-8fbe-4397-9dbb-9b2b20218538