DevTools: Generate InspectorBackend in the protocol externs

This is a preliminary step for splitting the externs

R=eustas, pfeldman, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/156963006

git-svn-id: svn://svn.chromium.org/blink/trunk@167092 bbb929c8-8fbe-4397-9dbb-9b2b20218538