[SVG] Implement 'discard' element

LayoutTests/svg/animations/script-tests/discard-on-discard.js

+description("Test the behavior of one discard applied on another discard");
+embedSVGTestCase("resources/discard-on-discard.svg");
+
+// Setup animation test
+function sample1() {
+    expectFillColor(rect1, 255, 0, 0);
+}
+
+function sample2() {
+    expectFillColor(rect1, 0, 255, 0);
+}
+
+function executeTest() {
+    var rects = rootSVGElement.ownerDocument.getElementsByTagName("rect");
+    rect1 = rects[0];
+
+    const expectedValues = [
+        // [animationId, time, sampleCallback]
+        ["anim", 0.0,   sample1],
+        ["anim", 0.01,   sample1],
+        ["anim", 2.0,   sample2],
+        ["anim", 2.01,   sample2],
+        ["anim", 3.0,   sample2],
+        ["anim", 3.01,   sample2]
+    ];
+
+    runAnimationTest(expectedValues);
+}
+
+window.animationStartsImmediately = true;
+var successfullyParsed = true;