Add assigner support to Blimp

- Add code to contact the assigner for an engine instance.
- Add plumbing to pipe the request from Java to native.
- Add plumbing to pipe the response back to Java to provide a UI
  indication of the success or failure of the request.
- Connect the assignment response with the BlimpClientSession.

BUG=582668

Review URL: https://codereview.chromium.org/1687393002

Cr-Commit-Position: refs/heads/master@{#376447}
21 files changed