Only use cross domain analytics on IER transactions

This is intentionally scoped to IER in a simple fashion as we decided not to build
a generalised solution until we have more than 1 example of user needs for this
kind of tracing, eg, do we have to support non-GA solutions, non-universal analytics?

Keep it simple for now.

https://www.pivotaltracker.com/n/projects/1008986/stories/74213668