update EffectsCallback signature

core-animation.html

@@ -51,7 +51,10 @@
 behaves essentially the same as `requestAnimationFrame`:
 
     var animation = new CoreAnimation();
-    animation.
+    animation.customEffect = function(timeFraction, target, animation) {
+      // do something custom
+    };
+    animation.play();
 
 Elements that are targets to a `core-animation` are given the `core-animation-target` class.
 
@@ -101,10 +104,11 @@
           keyframes: {value: null, reflect: true},
 
           /**
-           * A custom animation function. Either provide this or `keyframes`.
+           * A custom animation function. Either provide this or `keyframes`. The signature
+           * of the callback is EffectsCallback(timeFraction, target, animation)
            *
            * @property customEffect
-           * @type Function
+           * @type Function(number, Object, Object)
            */
           customEffect: {value: null, reflect: true},
 

demo.html

@@ -112,11 +112,11 @@
       <polymer-fadein>fade in</polymer-fadein>
  -->    </div>
     <script>
-      var customAnimationFn = function(timeFraction, animation, previousTimeFraction) {
+      var customAnimationFn = function(timeFraction, target) {
         if (timeFraction < 1) {
-          animation.target.textContent = timeFraction;
+          target.textContent = timeFraction;
         } else {
-          animation.target.textContent = 'animated!';
+          target.textContent = 'animated!';
         }
       };