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

components/script_bindings/codegen/Bindings.conf

@@ -799,6 +799,10 @@ Dictionaries = {
     'derives': ['Clone', 'Copy'],
 },
 
+'CSPViolationReportBody': {
+    'derives': ['Clone', 'MallocSizeOf'],
+},
+
 'FontFaceDescriptors': {
     'derives': ['Clone', 'MallocSizeOf']
 },
@@ -831,6 +835,14 @@ Dictionaries = {
     'derives': ['Clone'],
 },
 
+'Report': {
+    'derives': ['Clone', 'MallocSizeOf'],
+},
+
+'ReportBody': {
+    'derives': ['Clone', 'MallocSizeOf'],
+},
+
 'StereoPannerOptions': {
     'derives': ['Clone', 'Copy'],
 },