DevTools: [Elements] Discern shadow root types and differentiate them from document fragments in the UI

R=pfeldman, vsevik
BUG=234090

Review URL: https://codereview.chromium.org/65643008

git-svn-id: svn://svn.chromium.org/blink/trunk@162213 bbb929c8-8fbe-4397-9dbb-9b2b20218538