DevTools: Fix debug_devtools=1 mode "main" extensions loading after r182328

R=dgozman,lushnikov
NOTRY=true

Review URL: https://codereview.chromium.org/593673002

git-svn-id: svn://svn.chromium.org/blink/trunk@182405 bbb929c8-8fbe-4397-9dbb-9b2b20218538