Only execute default actions on trusted events.

LayoutTests/fast/dom/HTMLSelectElement/remove-element-from-within-focus-handler-crash.html

 <select id="bomb" onfocus="this.parentNode.removeChild(this)">
     <option>Clicking on this select element should not crash</option>
 </select>
 <h2>Layout test for <a href='https://bugs.webkit.org/show_bug.cgi?id=23858'>bug 23858</a></h2>
 <p>If this page is displayed without crashing then the test has passed.</p>
 <script>
     if (window.testRunner)
         testRunner.dumpAsText();
 
     var select = document.getElementById('bomb');
-    var mouseEvent = document.createEvent("MouseEvents");
-    mouseEvent.initMouseEvent("mousedown", true, true, document.defaultView, 1, select.offsetLeft + 1, select.offsetTop + 1, select.offsetLeft + 1, select.offsetTop + 1, false, false, false, false, 0, document);
-    select.dispatchEvent(mouseEvent);
+
+    var rect = select.getBoundingClientRect();
+    if (window.eventSender) {
+        eventSender.mouseMoveTo(rect.left + 1, rect.top + 1);
+        eventSender.mouseDown(0);
+    }
 </script>