Shadow: do not dispatch an event when target is reletedTarget

For example, when a mouse pointer moves from an element in a shadow tree to
its shadow host, a mouseover event should not be dispatched on the
shadow host. The current spec allows a non-empty event path in this case.

Fixes #337.

dom.bs

@@ -1217,6 +1217,9 @@ for discussion).
  <a>relatedTarget</a> against <var>target</var> if <var>event</var>'s <a>relatedTarget</a> is
  non-null, and null otherwise.
 
+ <li><p>If <var>target</var> is <var>relatedTarget</var> and <var>target</var> is not
+ <var>event</var>'s <a>relatedTarget</a>, then return true.
+
  <li><p>Append (<var>target</var>, <var>targetOverride</var>, <var>relatedTarget</var>) to <var>event</var>'s
  <a for=Event>path</a>.
 

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="2016-10-14">14 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="2016-10-18">18 October 2016</time></span></h2>
    </hgroup>
    <div data-fill-with="spec-metadata">
     <dl>
@@ -887,6 +887,8 @@ <h3 class="heading settled" data-level="3.8" id="dispatching-events"><span class
      <p class="note" role="note"><var>legacy target override flag</var> is only used by HTML and only when <var>target</var> is a <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/browsers.html#window">Window</a></code> object. </p>
     <li>Let <var>relatedTarget</var> be the result of <a data-link-type="dfn" href="#retarget">retargeting</a> <var>event</var>’s <a data-link-type="dfn" href="#event-relatedtarget">relatedTarget</a> against <var>target</var> if <var>event</var>’s <a data-link-type="dfn" href="#event-relatedtarget">relatedTarget</a> is
  non-null, and null otherwise. 
+    <li>
+     <p>If <var>target</var> is <var>relatedTarget</var> and <var>target</var> is not <var>event</var>’s <a data-link-type="dfn" href="#event-relatedtarget">relatedTarget</a>, then return true. </p>
     <li>
      <p>Append (<var>target</var>, <var>targetOverride</var>, <var>relatedTarget</var>) to <var>event</var>’s <a data-link-type="dfn" href="#event-path">path</a>. </p>
     <li>