Chrome Version       : 71.0.3578.98
OS Version: OS X 10.14.2
URLs (if applicable) :
Other browsers tested: chrome 70
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari:
    Firefox:
    IE/Edge:

What steps will reproduce the problem?

I'm gathering Report from ReportingObserver and Serialize them into JSON and sending bia Beacon API.

```js
const observer = new ReportingObserver((reports, observer) => {
    for (const report of reports) {
        console.log(JSON.stringify(report.body)) // {}
    }
}
```

What is the expected result?

Shows Serialized JSON text


What happens instead of that?

returns empty `{}` only


Please provide any additional information below. Attach a screenshot if
possible.


according to spec https://w3c.github.io/reporting/#concept-reports

> Each report has a body, which is either null or an object which can be serialized into a JSON text.

so this seems bug in serializer.


also report to here https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V1n3b8CyD1A


UserAgentString: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36


thanks

Jxck