Re: [web-animations-tests] Upstream negative-playback-rate.html, positive-playback-rate.html from Blink (#3327)

Also, as a meta point, as per the [README.md](https://github.com/w3c/web-platform-tests/blob/master/web-animations/README.md), as far as possible we should avoid adding helper methods that obscure what the test is doing, e.g. instead of adding

```diff
+function createIdleAnimation(t) {
+  var animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.reverse();
+  animation.cancel();
+  return animation;
+}
```

we should just write:

```js
test(function(t) {
  var animation = new Animation();
  assert_equals(animation.playState, 'idle');
}, "Play state is idle when currentTime is null");
```

View on GitHub: https://github.com/w3c/web-platform-tests/pull/3327#issuecomment-237148215

Received on Wednesday, 3 August 2016 05:51:13 UTC