Remove webkit prefix for RequestAnimationFrame

[rakudrama]

Per issue #4550:

RAF is sufficiently standardized to have an unprefixed API.

Remove these three members from the public API:

  void webkitCancelAnimationFrame(int id) native;
  void webkitCancelRequestAnimationFrame(int id) native;
  int webkitRequestAnimationFrame(RequestAnimationFrameCallback callback) native;

The names still need to be declared in impl type for dartium and dart2js

[sethladd]

Nice! :)

[efortuna]

Set owner to @efortuna.

[efortuna]

Fixed in revision 12926

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Added Fixed label.