DevTools: [Styles] Avoid click catching by .styles-clipboard-only elements

R=vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/259473005

git-svn-id: svn://svn.chromium.org/blink/trunk@172502 bbb929c8-8fbe-4397-9dbb-9b2b20218538