Steps to repro:

1) run MR integration tests with output enabled
2) see "Uncaught ReferenceError: media_router is not defined" message in output logs occasionally.

In the grand scheme of things it doesn't seem to affect the test result (maybe it is a race condition that was triggered by one of the ConditionalWait loops?) But it would be good to figure out what's causing this to reduce noise in our test logs.