Clear SVG Web Animation effects when animations exit their fill phase

third_party/WebKit/LayoutTests/animations/clear-svg-animation-effects.html

+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<svg>
+  <stop id="target" offset="1"/>
+</svg>
+<script>
+var asyncHandle = async_test('SVG Attributes should return to their base values after animations have stopped applying.');
+
+var animation = target.animate([
+  {svgOffset: '0'},
+  {svgOffset: '0.5'},
+], 1000);
+animation.pause();
+animation.currentTime = 500;
+
+requestAnimationFrame(() => {
+  asyncHandle.step(() => {
+    assert_equals(target.offset.animVal, 0.25, 'Animation is in effect.');
+  });
+  animation.cancel();
+  requestAnimationFrame(() => {
+    asyncHandle.step(() => {
+      assert_equals(target.offset.animVal, 1, 'Animation is no longer in effect.');
+    });
+    asyncHandle.done();
+  });
+});
+</script>