Make synthetic mouse events composed

Tests: see shadow-dom/event-composed.html in web-platform-tests.

Fixes WICG/webcomponents#578 and part of whatwg#1922.

source

@@ -3036,6 +3036,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x="dom-Event-currentTarget" data-x-href="https://dom.spec.whatwg.org/#dom-event-currenttarget"><code>currentTarget</code></dfn> attribute</li>
      <li><dfn data-x="dom-Event-bubbles" data-x-href="https://dom.spec.whatwg.org/#dom-event-bubbles"><code>bubbles</code></dfn> attribute</li>
      <li><dfn data-x="dom-Event-cancelable" data-x-href="https://dom.spec.whatwg.org/#dom-event-cancelable"><code>cancelable</code></dfn> attribute</li>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#composed-flag">composed flag</dfn></li>
      <li><dfn data-x="dom-Event-isTrusted" data-x-href="https://dom.spec.whatwg.org/#dom-event-istrusted"><code>isTrusted</code></dfn> attribute</li>
      <li><dfn data-x="dom-Event-initEvent" data-x-href="https://dom.spec.whatwg.org/#dom-event-initevent"><code>initEvent()</code></dfn> method</li>
      <li><dfn data-x="dom-EventTarget-addEventListener" data-x-href="https://dom.spec.whatwg.org/#dom-eventtarget-addeventlistener"><code>addEventListener()</code></dfn> method</li>
@@ -90064,6 +90065,8 @@ interface <dfn>DocumentAndElementEventHandlers</dfn> {
    <li><p>Initialize <var>event</var>'s <code data-x="dom-Event-bubbles">bubbles</code> and <code
    data-x="dom-Event-cancelable">cancelable</code> attributes to true.</p></li>
 
+   <li><p>Set <var>event</var>'s <span>composed flag</span>.</p></li>
+
    <li><p>If the <var>not trusted flag</var> is set, initialize <var>event</var>'s <code
    data-x="dom-Event-isTrusted">isTrusted</code> attribute to false.</p></li>