Web Animations API: Add simple keyframe tests

LayoutTests/web-animations-api/w3c/resources/keyframes-test.js

+(function(){
+'use strict'
+
+function createElement() {
+  var element = document.createElement('div');
+  document.documentElement.appendChild(element);
+  return element;
+}
+
+function heldTiming(iterationStart) {
+  return {
+    duration: 1000,
+    playbackRate: 0,
+    fill: 'forwards',
+    iterationStart: iterationStart,
+  };
+}
+
+function assertAnimationStyles(keyframes, expectations) {
+  for (var progress in expectations) {
+    var element = createElement();
+    element.animate(keyframes, heldTiming(progress));
+    var computedStyle = getComputedStyle(element);
+    for (var property in expectations[progress]) {
+      assert_equals(computedStyle[property], expectations[progress][property],
+          property + ' at ' + (progress * 100) + '%');
+    }
+  }
+}
+
+window.assertAnimationStyles = assertAnimationStyles;
+})();