[web-animations] Remove step about ignoring computedOffset when processing a keyframe object since it's not needed

This will only ever be needed if there is a CSS property computedOffset or
computed-offset and then we'll probably have bigger problems.

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extra : rebase_source : 4a2d6f077fd12f775cd82792f0b65c5d9b4def7b
[web-animations] Use [[Enumerate]] to iterate over the properties of keyframe objects

As suggested on public-script-coord: http://lists.w3.org/Archives/Public/public-script-coord/2014AprJun/0037.html

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extra : rebase_source : 753180bc4973eb11f54deeb97472617a3adb5e1f