Preserve running CSS Animations across changes to animation styles according to same name index

third_party/WebKit/LayoutTests/animations/multiple-same-animations-asan-crash.html

 <script src="../resources/testharness.js"></script>
 <script src="../resources/testharnessreport.js"></script>
 <style>
 #target {
   animation: test 1s, test 2s;
 }
 @keyframes test {
   to { color: green; }
 }
 </style>
 <div id="target"></div>
 <script>
 var asyncHandle = async_test('Don\'t crash ASAN when updating style with an element that has two animations with the same keyframes name and different timings.');
 // Wait a frame to let the animations get created.
 requestAnimationFrame(function() {
   // Force the next style recalc to be non-animation triggered.
   target.style.color = 'blue';
   requestAnimationFrame(function() {
-    // Clear the running animations to avoid hitting a debug assertion. (crbug.com/487092)
-    target.style.animation = 'none';
     asyncHandle.done();
   });
 });
 </script>