DevTools: [Elements] Improve shadow DOM representation

Shadow DOM is no longer dimmed and has a vertical bar to its left.

R=aandrey, eustas, pfeldman
BUG=305399

Review URL: https://codereview.chromium.org/75413009

git-svn-id: svn://svn.chromium.org/blink/trunk@163515 bbb929c8-8fbe-4397-9dbb-9b2b20218538