[SVG] Implement 'discard' element

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

Index: LayoutTests/svg/animations/discard-rect-as-href.svg
diff --git a/LayoutTests/svg/animations/discard-rect-as-href.svg b/LayoutTests/svg/animations/discard-rect-as-href.svg
new file mode 100644
index 0000000000000000000000000000000000000000..a44c889df9d69e5c8244f111155c935382d7d6e4
--- /dev/null
+++ b/LayoutTests/svg/animations/discard-rect-as-href.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" xlink:href="#r2" begin="2s"/>
+  <script>
+    if (window.testRunner)
+      testRunner.waitUntilDone();
+
+    function loaded() {
+      document.documentElement.setCurrentTime(2);
+
+      if (window.testRunner)
+        setTimeout(function(){testRunner.notifyDone();}, 10);
+    }
+  </script>
+</svg>