Support additive animations on CSS property transform

third_party/WebKit/LayoutTests/animations/interpolation/resources/interpolation-test.js

 /*
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 132 matching lines @@
           [property]: from,
         });
       }
       if (!isNeutralKeyframe(to)) {
         keyframes.push({
           offset: 1,
           composite: toComposite,
           [property]: to,
         });
       }
-      target.animate(keyframes, {
+      var animation = target.animate(keyframes, {
         fill: 'forwards',
         duration: 1,
         easing: createEasing(at),
-        delay: -0.5,
-        iterations: 0.5,
       });
+      animation.pause();
+      animation.currentTime = 0.5;
     },
     rebaseline: false,
   };
 
   function expectFlip(from, to, flipAt) {
     return [-0.3, 0, 0.3, 0.5, 0.6, 1, 1.5].map(function(at) {
       return {
         at: at,
         is: at < flipAt ? from : to
       };
@@ skipping 272 matching lines @@
       asyncHandle.done()
     });
   });
 
   window.assertInterpolation = assertInterpolation;
   window.assertNoInterpolation = assertNoInterpolation;
   window.assertComposition = assertComposition;
   window.afterTest = afterTest;
   window.neutralKeyframe = neutralKeyframe;
 })();