Add ReportingObserver API. (#6189)
Add ReportingObserver API. (#6189)

* Add ReportingObserver API.

* Update api/ReportingObserver.json

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data