Add ShadowRoot.prototype.delegatesFocus attribute

Tests: web-platform-tests/wpt#28593

Fixes #931.

dom.bs

@@ -5706,6 +5706,7 @@ invoked, must return a new {{DocumentFragment}} <a>node</a> whose <a for=Node>no
 [Exposed=Window]
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
+  readonly attribute boolean delegatesFocus;
   readonly attribute Element host;
   attribute EventHandler onslotchange;
 };
@@ -5738,6 +5739,9 @@ 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> attribute's getter must return
 <a>this</a>'s <a for=ShadowRoot>mode</a>.</p>
 
+<p>The <dfn attribute for=ShadowRoot><code>delegatesFocus</code></dfn> attribute's getter must
+return <a>this</a>'s <a for=ShadowRoot>delegates focus</a>.</p>
+
 <p>The <dfn attribute for=ShadowRoot><code>host</code></dfn> attribute's getter must return
 <a>this</a>'s <a for=DocumentFragment>host</a>.