GitLab

 - Stop passing the 'cr.webUIResponse' string from JS to C++ and again back to JS.
 - Restricting C++ response to JS to a single argument, since Promises
   can only be resolved with a single argument (this is also more consistent
   with Mojo-based communication).
 - Adding additional test cases.

BUG=580633

Review URL: https://codereview.chromium.org/1635673003

Cr-Commit-Position: refs/heads/master@{#371726}