fix($$RAFProvider): check for webkitCancelRequestAnimationFrame.

Request Type: bug

How to reproduce: Use AngularJS 1.2.14 together with ngAnimate in Cordova/PhoneGap WebView on Android 4.3

Component(s): ngAnimate

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

RAFProvider is checking for `cancelAnimationFrame` and `webkitCancelAnimationFrame` ( see https://github.com/angular/angular.js/blob/04d7317cdd95ba00783389f89f6e9a7e1fc418f8/src/ng/raf.js#L8 )
For Android 4.3 we need to check for `webkitCancelRequestAnimationFrame` too, otherwise the animations will fail.

**Other Comments:**

No breaking changes, closes #6526