[DevTools] Do not instantiate WebInspector.Toolbox in main.

Toolbox creates itself in a separate DOMContentLoaded listener.

BUG=391566,405084

Review URL: https://codereview.chromium.org/487343002

git-svn-id: svn://svn.chromium.org/blink/trunk@180629 bbb929c8-8fbe-4397-9dbb-9b2b20218538