Change Report and ReportBody interfaces to dicts.

This change removes any suggestion that the two interface names (and
the names of those interfaces which inherit from `ReportBody`) should
be exposed on the global object. `ReportBody` is purely abstract, and
exists so that it can be used as the type of the `body` member of
`Report`, while still allowing other specifications to define new
subtypes for their own report types.

Closes: #216

index.src.html

@@ -679,17 +679,13 @@ <h2 id="observers">Reporting Observers</h2>
   <h3 id=interface-reporting-observer>Interface {{ReportingObserver}}</h3>
 
   <pre class="idl">
-[Exposed=(Window,Worker)]
-interface ReportBody {
-  [Default] object toJSON();
+dictionary ReportBody {
 };
 
-[Exposed=(Window,Worker)]
-interface Report {
-  [Default] object toJSON();
-  readonly attribute DOMString type;
-  readonly attribute DOMString url;
-  readonly attribute ReportBody? body;
+dictionary Report {
+  DOMString type;
+  DOMString url;
+  ReportBody? body;
 };
 
 [Exposed=(Window,Worker)]
@@ -710,11 +706,11 @@ <h3 id=interface-reporting-observer>Interface {{ReportingObserver}}</h3>
 typedef sequence&lt;Report> ReportList;
   </pre>
 
-  A <dfn id=dom-report interface>Report</dfn> is the application exposed
-  representation of a <a>report</a>. <dfn attribute for="Report">type</dfn>
-  returns [=report/type=], <dfn attribute for="Report">url</dfn> returns
-  [=report/url=], and <dfn attribute for="Report">body</dfn> returns
-  [=report/body=].
+  A <dfn id=dom-report dictionary>Report</dfn> is the application-exposed
+  representation of a <a>report</a>.
+
+  <dfn id="reportbody" dictionary>ReportBody</dfn> is an abstract dictionary
+  type from which specific report types should inherit.
 
   Each {{ReportingObserver}} object has these associated concepts:
     - A <dfn for=ReportingObserver>callback</dfn> function set on creation.