[DevTools] Add Toolbox page to undocked DevTools frontend.

Toolbox page is the same frontend launched with "toolbox=true" parameter.
It has its own app, which creates a view hierarchy and reports to the main app.

Currently, toolbox is only created for responsive design.

BUG=327641

Review URL: https://codereview.chromium.org/298913004

git-svn-id: svn://svn.chromium.org/blink/trunk@175571 bbb929c8-8fbe-4397-9dbb-9b2b20218538