DevTools: [Elements] Allow editing of author shadow DOM

R=eustas, pfeldman, vsevik
BUG=348991

Review URL: https://codereview.chromium.org/196943010

git-svn-id: svn://svn.chromium.org/blink/trunk@169221 bbb929c8-8fbe-4397-9dbb-9b2b20218538