Remove document.origin

Use self.origin instead.

Tests: web-platform-tests/wpt#20995.

Closes #410.

Author: annevk

dom.bs

@@ -4747,7 +4747,6 @@ interface Document : Node {
   [SameObject] readonly attribute DOMImplementation implementation;
   readonly attribute USVString URL;
   readonly attribute USVString documentURI;
-  readonly attribute USVString origin;
   readonly attribute DOMString compatMode;
   readonly attribute DOMString characterSet;
   readonly attribute DOMString charset; // historical alias of .characterSet
@@ -4854,9 +4853,6 @@ null if <var>event</var>'s {{Event/type}} attribute value is "<code>load</code>"
  <dt><code><var>document</var> . {{Document/documentURI}}</code>
  <dd>Returns <var>document</var>'s <a for=Document>URL</a>.
 
- <dt><code><var>document</var> . {{Document/origin}}</code>
- <dd>Returns <var>document</var>'s <a for=Document>origin</a>.
-
  <dt><code><var>document</var> . {{Document/compatMode}}</code>
  <dd>
   Returns the string "<code>BackCompat</code>" if <var>document</var>'s
@@ -4887,10 +4883,6 @@ The <dfn attribute for=Document><code>URL</code></dfn> attribute's getter and
 <dfn attribute for=Document><code>documentURI</code></dfn> attribute's getter must return the
 <a for=Document>URL</a>, <a lt="URL serializer" spec=url>serialized</a>.
 
-The <dfn attribute for=Document><code>origin</code></dfn> attribute's getter must return the
-<a lt="serialization of an origin">serialization</a> of <a>context object</a>'s
-<a for=Document>origin</a>.
-
 The <dfn attribute for=Document><code>compatMode</code></dfn> attribute's getter must
 return "<code>BackCompat</code>" if <a>context object</a>'s <a for=Document>mode</a> is
 "<code>quirks</code>", and "<code>CSS1Compat</code>" otherwise.