DevTools: use extensions for SearchScopes

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165313

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/135903008

git-svn-id: svn://svn.chromium.org/blink/trunk@165397 bbb929c8-8fbe-4397-9dbb-9b2b20218538