DevTools: fix author shadow dom inspection mode, speacial-case ua. Fix crash from the bug.

R=apavlov
BUG=353016

Review URL: https://codereview.chromium.org/197283031

git-svn-id: svn://svn.chromium.org/blink/trunk@169441 bbb929c8-8fbe-4397-9dbb-9b2b20218538