DevTools: Get rid of Element.prototype.enableStyleClass

R=pfeldman, apavlov, eustas
NOTRY=true

Review URL: https://codereview.chromium.org/182113004

git-svn-id: svn://svn.chromium.org/blink/trunk@168442 bbb929c8-8fbe-4397-9dbb-9b2b20218538