DevTools: Fix "toolbox" app startup in the debug_devtools mode

This was broken by the async module loading.

R=pfeldman@chromium.org,dgozman

Review URL: https://codereview.chromium.org/648153002

git-svn-id: svn://svn.chromium.org/blink/trunk@183599 bbb929c8-8fbe-4397-9dbb-9b2b20218538