Add "same origin with its ancestors"

See discussion in w3c/webappsec-credential-management#253.

source

@@ -96471,11 +96471,6 @@ interface <dfn interface>NotRestoredReasons</dfn> {
    <li><p>Return <var>navigables</var>.</p></li>
   </ol>
 
-  <p class="note">These descendant-collecting algorithms are described as looking at the DOM tree
-  of descendant <code>Document</code> objects. In reality, this is often not feasible since the DOM
-  tree can be in another process from the caller of the algorithm. Instead, implementations
-  generally replicate the appropriate trees across processes.</p>
-
   <p id="document-tree-child-browsing-context">The <dfn>document-tree child navigables</dfn> of a
   <code>Document</code> <var>document</var> are given by these steps:</p>
 
@@ -96505,6 +96500,35 @@ interface <dfn interface>NotRestoredReasons</dfn> {
    <li><p>Return <var>navigables</var>.</p></li>
   </ol>
 
+  <p class="note">These descendant-collecting algorithms are described as looking at the DOM tree
+  of descendant <code>Document</code> objects. In reality, this is often not feasible since the DOM
+  tree can be in another process from the caller of the algorithm. Instead, implementations
+  generally replicate the appropriate trees across processes.</p>
+
+  <p>A <code>Document</code> <var>document</var> is <dfn export>same origin with its
+  ancestors</dfn> if the following algorithm returns true:</p>
+
+  <ol>
+   <li><p>Let <var>origin</var> be <var>document</var>'s <span
+   data-x="concept-document-origin">origin</span>.</p></li>
+
+   <li>
+    <p><span data-x="list iterate">For each</span> <var>ancestorNavigable</var> of
+    <var>document</var>'s <span>ancestor navigables</span>:</p>
+
+    <ol>
+     <li><p>Let <var>ancestorOrigin</var> be <var>ancestorNavigable</var>'s <span
+     data-x="nav-document">document</span>'s <span
+     data-x="concept-document-origin">origin</span>.</p></li>
+
+     <li><p>If <var>ancestorOrigin</var> is not <span>same origin</span> with <var>origin</var>,
+     then return false.</p></li>
+    </ol>
+   </li>
+
+   <li><p>Return true.</p></li>
+  </ol>
+
 
   <h5 id="garbage-collection-and-browsing-contexts">Navigable destruction</h5>