[DevTools] Move user agent override to responsive design toolbar.

This also made toolbar expandable to accomodate controls for rarely used features.

BUG=327641

Review URL: https://codereview.chromium.org/317153002

git-svn-id: svn://svn.chromium.org/blink/trunk@175663 bbb929c8-8fbe-4397-9dbb-9b2b20218538