Implement initial version of ReportingObserver (#37905)

The specification moved around lately with how it defines its reports
and report bodies. They became dictionaries, but are currently missing
some fields [1].

Most tests won't be passing yet, since the `Reporting-Endpoints` header
isn't used yet. In fact, the specification leaves it up to the browser
to figure out when to run this task [2]. I am not sure if there some
background scheduling we can do here.

Confirmed with content-security-policy/reporting-api/
report-to-directive-allowed-in-meta.https.sub.html that the callback is
invoked. The test doesn't pass, since
the `describe_scripted_caller` is empty for HTML elements. Thus the
`source_file` is empty, whereas it should be equivalent to the current
document URL.

Part of #37328

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

[1]: https://github.com/w3c/reporting/issues/286
[2]: https://w3c.github.io/reporting/#report-delivery

tests/wpt/mozilla/meta/MANIFEST.json

@@ -13640,14 +13640,14 @@
      ]
     ],
     "interfaces.https.html": [
-     "eee8c799727b91e00b512795756b693a5f121f86",
+     "8a9345525360e7a7ce69e84e394b65a4cbc0ab34",
      [
       null,
       {}
      ]
     ],
     "interfaces.worker.js": [
-     "e86f34f261442aeaa7074c525fb4b1206219769d",
+     "f217fd8fb6b46144bc3576a081cc6ce5db3129d5",
      [
       "mozilla/interfaces.worker.html",
       {}