Discussion: https://www.w3.org/2016/11/30-webperf-minutes.html

Background: #55. In #79 we added a note to flag the implementation differences across browsers. Ideally, we should aim to converge on a consistent implementation:

1. It would be great to get some interop tests to identify where/how existing implementations differ on whether the entry is made available before or after the load event.
2. Real-world telemetry on how often developers rely on querying entry from within onload would, also, be very helpful.