DevTools: Fix "Show editor in drawer" experiment

This regressed in r166725

R=vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/157803009

git-svn-id: svn://svn.chromium.org/blink/trunk@166908 bbb929c8-8fbe-4397-9dbb-9b2b20218538