Make Document origin alias its policy container's origin.

This change depends on the changes in whatwg/html#8447
which adds origin to policy container.

dom.bs

@@ -4337,8 +4337,9 @@ dom-Range-extractContents, dom-Range-cloneContents -->
   <dl class=switch>
    <dt>{{Document}}
    <dd><p>Set <var>copy</var>'s <a for=Document>encoding</a>, <a for=Document>content type</a>,
-   <a for=Document>URL</a>, <a for=Document>origin</a>, <a for=Document>type</a>, and
-   <a for=Document>mode</a> to those of <var>node</var>.
+   <a for=Document>URL</a>, <a for=Document>policy container</a>'s
+   <a for="Policy Container">origin</a>, <a for=Document>type</a>, and <a for=Document>mode</a>
+   to those of <var>node</var>.
 
    <dt>{{DocumentType}}
    <dd><p>Set <var>copy</var>'s <a for=DocumentType>name</a>, <a>public ID</a>, and
@@ -4880,8 +4881,8 @@ known as <dfn export id=concept-document lt="document">documents</dfn>.
 <p>Unless stated otherwise, a <a>document</a>'s <a for=Document>encoding</a> is the <a>utf-8</a>
 <a for=/>encoding</a>, <a for=Document>content type</a> is
 "<code>application/xml</code>", <a for=Document>URL</a> is "<code>about:blank</code>",
-<a for=Document>origin</a> is an <a>opaque origin</a>,
-<a for=Document>type</a> is "<code>xml</code>", and its
+<a for=Document>origin</a> is the <a>document</a>'s <a for=Document>policy container</a>'s
+<a for="Policy Container">origin</a>, <a for=Document>type</a> is "<code>xml</code>", and its
 <a for=Document>mode</a> is "<code>no-quirks</code>".
 
 <p>A <a>document</a> is said to be an <dfn export>XML document</dfn> if its <a for=Document>type</a>
@@ -4940,7 +4941,8 @@ null if <var>event</var>'s {{Event/type}} attribute value is "<code>load</code>"
 </dl>
 
 <p>The <dfn constructor for=Document lt=Document()><code>new Document()</code></dfn> constructor
-steps are to set <a>this</a>'s <a for=Document>origin</a> to the <a for=Document>origin</a> of
+steps are to set <a>this</a>'s <a for=Document>policy container</a>'s
+<a for="Policy Container">origin</a> to the <a for=Document>origin</a> of
 <a>current global object</a>'s <a>associated <code>Document</code></a>. [[!HTML]]
 
 <p class=note>Unlike {{DOMImplementation/createDocument()}}, this constructor does not