Shadow: add DocumentOrShadowRoot mixin

See WICG/webcomponents#91 for context.

dom.bs

@@ -1898,6 +1898,22 @@ method, when invoked, must return the first <a for="/">element</a>, in <a>tree o
 there is no such <a for="/">element</a> otherwise.
 
 
+<h4 id=mixin-documentorshadowroot>Mixin {{DocumentOrShadowRoot}}</h4>
+
+<pre class=idl>
+[NoInterfaceObject,
+ Exposed=Window]
+interface DocumentOrShadowRoot {
+};
+Document implements DocumentOrShadowRoot;
+ShadowRoot implements DocumentOrShadowRoot;
+</pre>
+
+<p class="note no-backref">The {{DocumentOrShadowRoot}} mixin is expected to be used by other
+standards that want to define APIs shared between <a for=/>documents</a> and
+<a for=/>shadow roots</a>.
+
+
 <h4 id=interface-parentnode>Mixin {{ParentNode}}</h4>
 
 To