[SVG] Handle beginEvent for svg animations

LayoutTests/svg/animations/begin-event.svg

Index: LayoutTests/svg/animations/begin-event.svg
diff --git a/LayoutTests/svg/animations/begin-event.svg b/LayoutTests/svg/animations/begin-event.svg
new file mode 100755
index 0000000000000000000000000000000000000000..5851eff23d5c632abb26e8b1e18c3c0abc3a88b0
--- /dev/null
+++ b/LayoutTests/svg/animations/begin-event.svg
@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1 Tiny//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11-tiny.dtd">
+<svg onload="loaded()" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+  <rect x="0" y="0" width="0" height="100" fill="green">
+    <animate id="anim" attributeName="visibility" to="visible" begin="0s" end="2s" fill="freeze"/>
+    <set attributeName="width" to="100" begin="anim.beginEvent" />
+  </rect>
+  <script>
+    if (window.testRunner)
+        testRunner.waitUntilDone();
+
+    function loaded() {
+        document.documentElement.setCurrentTime(2);
+
+        if (window.testRunner)
+            setTimeout(function(){testRunner.notifyDone();}, 100);
+    }
+  </script>
+</svg>
+