DevTools: Decouple AdvancedApp from Toolbox

This is required to introduce a new "toolbox" application
within the new application loading scheme.

R=dgozman, pfeldman, vsevik
BUG=391566

Review URL: https://codereview.chromium.org/460673003

git-svn-id: svn://svn.chromium.org/blink/trunk@180178 bbb929c8-8fbe-4397-9dbb-9b2b20218538