Add RTCPeerConnection constructor error to spec.

spec.bs

@@ -2474,6 +2474,25 @@ nested traversables in [[HTML]]'s <a
 href=https://html.spec.whatwg.org/multipage/document-sequences.html#top-level-traversables>Top-level
 traversables</a> section.
 
+<div algorithm>
+  To get the <dfn for=navigable>fenced navigable ancestor</dfn> of a [=navigable=] |navigable|:
+
+  1. Let |currentNavigable| be |navigable|. 
+
+  1. While |currentNavigable| is not null:
+
+      1. If |currentNavigable| is a [=fenced navigable container/fenced navigable=], return 
+         |currentNavigable|.
+
+      1. Set |currentNavigable| to |currentNavigable|'s [=navigable/parent=].
+
+  1. Return |currentNavigable|.
+
+  Note: Some Web Platform algorithms will have additional restrictions if the 
+  [=relevant global object=]'s [=Window/navigable=] has a [=navigable/fenced navigable ancestor=]. 
+  For an example, see the [[#webrtc-monkeypatch]] section below.     
+</div>
+
 <h4 id=top-level-traversables>Top-level traversables</h4>
 
 The [[HTML]] Standard currently defines a [=top-level traversable=] as a [=traversable navigable=]
@@ -3918,6 +3937,24 @@ at the expense of some utility.
   </wpt>
 </div>
 
+<h3 id=webrtc-monkeypatch>WebRTC</h3>
+
+The [[WEBRTC]] specification defines "ECMAScript APIs in WebIDL to allow media and generic 
+application data to be sent to and received from another browser or device implementing the 
+appropriate set of real-time protocols." The interface which facilitates connections to peers
+is {{RTCPeerConnection}}. Construction of this interface, and therefore connection to peers 
+via WebRTC, is disallowed in fenced frames.
+
+<div algorithm=webrtc-constructor>
+  Modify the {{RTCPeerConnection}} {{RTCPeerConnection/constructor}} algorithm to add new first and
+  second steps that read:
+
+  1. Let |navigable| be [=this=]'s [=relevant global object=]'s [=Window/navigable=].
+
+  1. If |navigable| has a [=navigable/fenced navigable ancestor=], throw a {{NotAllowedError}}
+     {{DOMException}}. 
+</div>
+
 <h2 id=security-and-privacy>Security & Privacy Considerations</h2>
 
 This material is being upstreamed from our explainer into this specification, and in the meantime