Web Animations: support cssOffset

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 118 matching lines @@
       this.interpolateComposite(property, from, 'replace', to, 'replace', at, target);
     },
     interpolateComposite: function(property, from, fromComposite, to, toComposite, at, target) {
       // Convert standard properties to camelCase.
       if (!property.startsWith('--')) {
         for (var i = property.length - 2; i > 0; --i) {
           if (property[i] === '-') {
             property = property.substring(0, i) + property[i + 1].toUpperCase() + property.substring(i + 2);
           }
         }
+        if (property === 'offset')
+          property = 'cssOffset';
       }
       var keyframes = [];
       if (!isNeutralKeyframe(from)) {
         keyframes.push({
           offset: 0,
           composite: fromComposite,
           [property]: from,
         });
       }
       if (!isNeutralKeyframe(to)) {
@@ skipping 317 matching lines @@
       asyncHandle.done()
     });
   });
 
   window.assertInterpolation = assertInterpolation;
   window.assertNoInterpolation = assertNoInterpolation;
   window.assertComposition = assertComposition;
   window.afterTest = afterTest;
   window.neutralKeyframe = neutralKeyframe;
 })();