My idea that there could be a race between getting the spare renderer, and the spare renderer getting killed, does not pan out; as far as I can see the spare renderer won't be canceled except in an explicit way which can't happen in the test.

At any rate, the test has stopped flaking according to the dashboard, so closing:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=chrome_public_test_apk&tests=testCellularPrerenderingDoesntKillSpareRenderer