DevTools: show styles for shadow dom elements

This patch fixes a bug which prevented devTools from showing styles
for shadow dom elements.

BUG=279390

Review URL: https://codereview.chromium.org/23629018

git-svn-id: svn://svn.chromium.org/blink/trunk@160328 bbb929c8-8fbe-4397-9dbb-9b2b20218538