Don't expose the user agent shadow tree

LayoutTests/svg/custom/use-event-handler-on-referenced-element-hierarchy.svg

+<svg version="1.1" baseProfile="basic" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="clickRect()">
+  <script xlink:href="../../fast/repaint/resources/text-based-repaint.js"/>
+  <defs id="hidden">
+    <g id="reference">
+      <rect onclick="test(evt)" id="referenceRect" x="240" y="1" width="239" height="358"/>
+    </g>
+
+   <script type="text/ecmascript">
+    <![CDATA[
+    if (window.testRunner)
+        testRunner.dumpAsText();
+
+    function clickRect() {
+        if (window.eventSender) {
+            testRunner.waitUntilDone();
+            eventSender.mouseMoveTo(250, 50);
+            eventSender.mouseDown();
+            eventSender.mouseUp();
+        }
+    }
+
+    function test(evt) {
+        var rect = evt.currentTarget;
+        var g = document.getElementById("reference");
+        var defs = document.getElementById("hidden");
+        if (rect.parentNode == g && rect.parentNode.parentNode == defs)
+            document.getElementById("text").firstChild.nodeValue = "Test passed.";
+        if (window.testRunner)
+            testRunner.notifyDone();
+    }
+    ]]>
+    </script>
+  </defs>
+
+  <use xlink:href="#reference" fill="grey"/>
+
+  <text id="text" transform="translate(5,20)">Test failed.</text>
+  <rect id="test-frame" x="1" y="1" width="478" height="358" fill="none" stroke="#000"/>
+</svg>