DevTools: [Elements] Do not add editing-related context menu items for edited nodes

R=eustas, lushnikov, pfeldman
BUG=358098
NOTRY=true

Review URL: https://codereview.chromium.org/220763004

git-svn-id: svn://svn.chromium.org/blink/trunk@170562 bbb929c8-8fbe-4397-9dbb-9b2b20218538