Shadow: safelist "main" in attachShadowRoot()

Fixes WICG/webcomponents#593.

dom.bs

@@ -6408,6 +6408,7 @@ invoked, must run these steps:
  "<code>h5</code>",
  "<code>h6</code>",
  "<code>header</code>",
+ "<code>main</code>"
  "<code>nav</code>",
  "<code>p</code>",
  "<code>section</code>", or
@@ -9828,6 +9829,7 @@ Cameron McCormack,
 Chris Dumez,
 Chris Paris,
 Chris Rebert,
+Cyrille Tuzi,
 Daniel Glazman,
 Darin Fisher,
 David Bruant,

dom.html

@@ -71,7 +71,7 @@
    <p data-fill-with="logo"><a class="logo" href="https://whatwg.org/"> <img alt="WHATWG" height="100" src="https://resources.whatwg.org/logo-dom.svg"> </a> </p>
    <hgroup>
     <h1 class="p-name no-ref allcaps" id="title">DOM</h1>
-    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2016-10-20">20 October 2016</time></span></h2>
+    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2016-10-24">24 October 2016</time></span></h2>
    </hgroup>
    <div data-fill-with="spec-metadata">
     <dl>
@@ -3978,6 +3978,7 @@ <h3 class="heading settled" data-level="4.9" id="interface-element"><span class=
  "<code>h5</code>",
  "<code>h6</code>",
  "<code>header</code>",
+ "<code>main</code>"
  "<code>nav</code>",
  "<code>p</code>",
  "<code>section</code>", or
@@ -5541,6 +5542,7 @@ <h2 class="no-num heading settled" id="acks"><span class="content">Acknowledgmen
 Chris Dumez,
 Chris Paris,
 Chris Rebert,
+Cyrille Tuzi,
 Daniel Glazman,
 Darin Fisher,
 David Bruant,