Clear event's target and relatedTarget when dispatch ends at a shadow root

Tests: web-platform-tests/wpt#7481.

Fixes #511.

Author: Takayoshi Kochi

dom.bs

@@ -1253,6 +1253,9 @@ for discussion).
 
  <li><p>Set <var>event</var>'s {{Event/eventPhase}} attribute to {{Event/NONE}}.
 
+ <li><p>If {{Event/target}}'s <a for=tree>root</a> is a <a for=/>shadow root</a>, then set
+ <var>event</var>'s {{Event/target}} attribute and <var>event</var>'s <a>relatedTarget</a> to null.
+
  <li><p>Set <var>event</var>'s {{Event/currentTarget}} attribute to null.
 
  <li><p>Set <var>event</var>'s <a for=Event>path</a> to the empty list.