AI 144044: First cut of tunneling HTTP requests over gtalk connection:

  - We define HttpRequest and HttpResponse protocol buffers.
  The RMQ mechanism is *not* used for these (though the
  messages define rmq id fields in case we want to start).
  - GTalkHttpClient will send an http request over the
  connection to MCS if:
  - gtalk_tunnel_http in gservices is turned on
  - the request destination is the android proxy
  - it can make a connection to gtalkservice
  The request falls back to using GoogleHttpClient if it can't
  be processed over the MCS connection.
  A few shortcuts are taken with cookie handling, but these
  are okay for the envisioned usage.

Automated import of CL 144044