Handle shadow DOM and <dialog> focusing

Closes whatwg#2393, by making <dialog> behave the same as delegatesFocus: they both "delegate focus" to either autofocus="" if it's specified on a (non-shadow-including) descendant, or the first focusable element among their shadow-including descendants if not.

source

@@ -58171,16 +58171,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   <ol>
    <li><p>If <var>subject</var> is <span>inert</span>, return.</p></li>
 
-   <li>
-    <p>Let <var>control</var> be the first descendant element of <var>subject</var>, in <span>tree
-    order</span>, that is not <span>inert</span> and has the <code
-    data-x="attr-fe-autofocus">autofocus</code> attribute specified.</p>
-
-    <p>If there isn't one, then let <var>control</var> be the first non-<span>inert</span>
-    descendant element of <var>subject</var>, in tree order.
+   <li><p>Let <var>control</var> be the <span>focus delegate</span> of <var>subject</var>.</p></li>
 
-    <p>If there isn't one of those either, then let <var>control</var> be <var>subject</var>.</p>
-   </li>
+   <li><p>If <var>control</var> is null, then set <var>control</var> to <var>subject</var>.</p></li>
 
    <li>
     <p>Run the <span>focusing steps</span> for <var>control</var>.</p>
@@ -75189,27 +75182,8 @@ END:VCARD</pre>
      <span>currently focused area of a top-level browsing context</span>'s <span>DOM anchor</span>,
      then return null.</p></li>
 
-     <li><p>Let <var>autofocus delegate</var> be the <span>autofocus delegate</span> for
-     <var>root</var> given <var>focus trigger</var>.</p></li>
-
-     <li><p>If <var>autofocus delegate</var> is not null, then return <var>autofocus
-     delegate</var>.</p></li>
-
-     <li><p>If <var>focus trigger</var> is "<code data-x="">click</code>", then let <var>possible
-     focus delegates</var> be the list of all <span>click focusable</span> <span data-x="focusable
-     area">focusable areas</span> whose <span>DOM anchor</span> is a <span>shadow-including
-     descendant</span> of <var>root</var>.</p></li>
-
-     <li><p>Otherwise, let <var>possible focus delegates</var> be the list of all <span
-     data-x="focusable area">focusable areas</span> whose <span>DOM anchor</span> is a
-     <span>shadow-including descendant</span> of <var>root</var>.</p></li>
-
-     <li><p>If <var>possible focus delegates</var> is empty, then return null.</p></li>
-
-     <li><p>Sort <var>possible focus delegates</var> by <span>shadow-including tree
-     order</span> of their <span data-x="DOM anchor">DOM anchors</span>.</p></li>
-
-     <li><p>Return <var>possible focus delegates</var>[0].</p></li>
+     <li><p>Return the <span>focus delegate</span> for <var>root</var> given <var>focus
+     trigger</var>.</p></li>
     </ol>
 
     <p class="note">For <span data-x="sequentially focusable">sequential focusability</span>, the
@@ -75224,6 +75198,34 @@ END:VCARD</pre>
    <dd><p>Return null.</p></dd>
   </dl>
 
+  <p>The <dfn>focus delegate</dfn> for a <var>focus target</var>, given an optional string
+  <var>focus trigger</var> (default "<code data-x="">other</code>"), is given by the following
+  steps:</p>
+
+  <ol>
+   <li><p>Let <var>autofocus delegate</var> be the <span>autofocus delegate</span> for
+   <var>focus target</var> given <var>focus trigger</var>.</p></li>
+
+   <li><p>If <var>autofocus delegate</var> is not null, then return <var>autofocus
+   delegate</var>.</p></li>
+
+   <li><p>If <var>focus trigger</var> is "<code data-x="">click</code>", then let <var>possible
+   focus delegates</var> be the list of all <span>click focusable</span> <span data-x="focusable
+   area">focusable areas</span> whose <span>DOM anchor</span> is a <span>shadow-including
+   descendant</span> of <var>focus target</var>.</p></li>
+
+   <li><p>Otherwise, let <var>possible focus delegates</var> be the list of all <span
+   data-x="focusable area">focusable areas</span> whose <span>DOM anchor</span> is a
+   <span>shadow-including descendant</span> of <var>focus target</var>.</p></li>
+
+   <li><p>If <var>possible focus delegates</var> is empty, then return null.</p></li>
+
+   <li><p>Sort <var>possible focus delegates</var> by <span>shadow-including tree order</span> of
+   their <span data-x="DOM anchor">DOM anchors</span>.</p></li>
+
+   <li><p>Return <var>possible focus delegates</var>[0].</p></li>
+  </ol>
+
   <p>The <dfn>autofocus delegate</dfn> for a <var>focus target</var> given a <var>focus
   trigger</var> is given by the following steps:</p>