Add ShadowRoot.prototype.delegatesFocus attribute

Tests: web-platform-tests/wpt#28593.

Fixes #931.

dom.bs

@@ -5697,6 +5697,7 @@ constructor steps are to set <a>this</a>'s <a for=Node>node document</a> to
 [Exposed=Window]
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
+  readonly attribute boolean delegatesFocus;
   readonly attribute SlotAssignmentMode slotAssignment;
   readonly attribute Element host;
   attribute EventHandler onslotchange;
@@ -5734,6 +5735,12 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <p>The <dfn attribute for=ShadowRoot><code>mode</code></dfn> getter steps are to return
 <a>this</a>'s <a for=ShadowRoot>mode</a>.</p>
 
+<p>The <dfn attribute for=ShadowRoot><code>delegatesFocus</code></dfn> getter steps are to return
+<a>this</a>'s <a for=ShadowRoot>delegates focus</a>.</p>
+
+<p>The <dfn attribute for=ShadowRoot><code>slotAssignment</code></dfn> getter steps are to return
+<a>this</a>'s <a for=ShadowRoot>slot assignment</a>.
+
 <p>The <dfn attribute for=ShadowRoot><code>host</code></dfn> getter steps are to return
 <a>this</a>'s <a for=DocumentFragment>host</a>.
 
@@ -5742,9 +5749,6 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <dfn for=ShadowRoot export><code>onslotchange</code></dfn> <a>event handler</a>, whose
 <a>event handler event type</a> is {{HTMLSlotElement/slotchange}}.
 
-<p>The <dfn attribute for=ShadowRoot><code>slotAssignment</code></dfn> getter steps are to return
-<a>this</a>'s <a for=ShadowRoot>slot assignment</a>.
-
 <hr>
 
 <p>In <dfn export id=concept-shadow-including-tree-order>shadow-including tree order</dfn> is