[SVG] Implement 'discard' element

LayoutTests/svg/animations/discard-check-delete.svg

+<?xml version="1.0" encoding="UTF-8"?>
+<svg onload="loaded()" version="1.2" baseProfile="tiny" id="svg-root" width="100%" height="100%"
+  viewBox="0 0 480 360" xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink">
+  <rect id="r1" x="100" y="100" width="200" height="200" fill="green"/>
+  <rect id="r2" x="100" y="100" width="200" height="200" fill="red" />
+  <discard id="discard1" xlink:href="#r2" begin="2s"/>
+  <set id="set" attributeName="display" to="inline" begin="3s" dur="1s" fill="freeze" onbegin="test();"/>
+  <script>
+    if (window.testRunner)
+      testRunner.waitUntilDone();
+
+    function loaded() {
+      document.documentElement.setCurrentTime(4);
+
+      if (window.testRunner)
+        setTimeout(function(){testRunner.notifyDone();}, 100);

[pdr., 2013/12/20 23:49:11]

Most of these setTimeout(...) calls can have values less than 100 for the delay.
I think 0 may work, or 20 or 30.

+    }
+
+    function test() {
+      var d1 = document.getElementById("discard1");
+      var r2 = document.getElementById("r2");
+
+      if(d1 || r2)
+        document.getElementById("r1").setAttribute("fill", "red");
+    }
+  </script>
+</svg>