Add delegatesFocus to attachShadow and flag to ShadowRoot

Fixes #367, i.e. the DOM part of
whatwg/html#2013. Further commits in HTML will
use this flag.

dom.bs

@@ -5764,6 +5764,9 @@ enum ShadowRootMode { "open", "closed" };
 <p><a for=/>Shadow roots</a> have an associated <dfn for=ShadowRoot>mode</dfn> ("<code>open</code>"
 or "<code>closed</code>").</p>
 
+<p><a for=/>Shadow roots</a> have an associated <dfn export for=ShadowRoot>delegates focus</dfn>.
+It is initially set to false.</p>
+
 <p><a for=/>Shadow roots</a>'s associated <a for=DocumentFragment>host</a> is never null.</p>
 <!-- If we ever change this, e.g., add a ShadowRoot object constructor, that would have serious
      consequences for innerHTML. -->
@@ -5907,6 +5910,7 @@ interface Element : Node {
 
 dictionary ShadowRootInit {
   required ShadowRootMode mode;
+  boolean delegatesFocus = false;
 };
 </pre>
 
@@ -6805,6 +6809,9 @@ invoked, must run these steps:
  is <a>context object</a>'s <a for=Node>node document</a>, <a for=DocumentFragment>host</a> is
  <a>context object</a>, and <a for=ShadowRoot>mode</a> is <var>init</var>'s {{ShadowRootInit/mode}}.
 
+ <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>delegates focus</a> to <var>init</var>'s
+ {{ShadowRootInit/delegatesFocus}}.
+
  <li><p>Set <a>context object</a>'s <a for=Element>shadow root</a> to <var>shadow</var>.
 
  <li><p>Return <var>shadow</var>.
@@ -10217,6 +10224,7 @@ prosody—Gab<!-- riel --> Vereable<!-- Gaston --> <!-- Croft -->Context(,
 Pierre-Marie Dartus,
 Ra'Shaun Stovall (Snuggs),
 Rafael Weinstein,
+Rakina Zata Amni,
 Richard Bradshaw,
 Rick Byers,
 Rick Waldron,