DevTools: fix new rule default style selector

This patch sets simple node selector instead of optimized csspath for
newly added rule.

BUG=347015

Review URL: https://codereview.chromium.org/176873014

git-svn-id: svn://svn.chromium.org/blink/trunk@168020 bbb929c8-8fbe-4397-9dbb-9b2b20218538