Remove document.origin

Use self.origin instead. (The document's origin internal concept is kept for now. Maybe at some point we can only use that of the relevant global object, but not now.)

Tests: ...

Closes #410.

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.