Release Deprecate AnimationTiming.playbackRate and fix for iOS 7 · web-animations/web-animations-js

Fix a critical bug on iOS 7 and Safari <= 6. Due to limitations,
inline style patching is not supported on these platforms.
Deprecated AnimationTiming.playbackRate.

For example, this is no longer supported:
```
var player = element.animate(
    keyframes,
    {duration: 1000, playbackRate: 2});
```
Use AnimationPlayer.playbackRate instead:
```
var player = element.animate(
    keyframes,
    {duration: 1000});
player.playbackRate = 2;
```
If you have any feedback on this change, please start a discussion
on the public-fx mailing list:
http://lists.w3.org/Archives/Public/public-fx/

Or file an issue against the specification on GitHub:
https://github.com/w3c/web-animations/issues/new

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