DevTools: Use "toolbox.html" in URL passed to window.open() as a toolbox

This is the Blink part of a two-sided change.
The Chromium part is https://codereview.chromium.org/620953003

R=dgozman, vsevik

Review URL: https://codereview.chromium.org/615893004

git-svn-id: svn://svn.chromium.org/blink/trunk@183133 bbb929c8-8fbe-4397-9dbb-9b2b20218538