DevTools: Allow general editing of author shadow DOM elements

As a drive-by, shadow DOM editing tests have been extracted into a new HTML

R=aandrey, pfeldman, vsevik
BUG=353910
NOTRY=true

Review URL: https://codereview.chromium.org/206503002

git-svn-id: svn://svn.chromium.org/blink/trunk@169743 bbb929c8-8fbe-4397-9dbb-9b2b20218538