[SVG] Implement 'discard' element

LayoutTests/svg/animations/discard-rect-as-child.svg

Index: LayoutTests/svg/animations/discard-rect-as-child.svg
diff --git a/LayoutTests/svg/animations/discard-rect-as-child.svg b/LayoutTests/svg/animations/discard-rect-as-child.svg
new file mode 100644
index 0000000000000000000000000000000000000000..15aefdcc348a41be14ed84f356e0a9ce5b243ad3
--- /dev/null
+++ b/LayoutTests/svg/animations/discard-rect-as-child.svg
@@ -0,0 +1,20 @@
+<?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" begin="2s"/>
+  </rect>
+  <script>
+    if (window.testRunner)
+      testRunner.waitUntilDone();
+
+    function loaded() {
+      document.documentElement.setCurrentTime(2);
+
+      if (window.testRunner)
+        setTimeout(function(){testRunner.notifyDone();}, 10);
+    }
+  </script>
+</svg>