DevTools: Display <return> value in Local scope's extra parameters.

This depends on https://codereview.chromium.org/50043005/

BUG=155730
R=yurys@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/48113027

git-svn-id: svn://svn.chromium.org/blink/trunk@161339 bbb929c8-8fbe-4397-9dbb-9b2b20218538