Auto merge of #14899 - canaltinova:transition-timing, r=Manishearth

Fix serialization of timing-function

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`transition-timing-function` and `animation-timing-function` properties corrected to reflect [the spec](https://drafts.csswg.org/css-transitions/#serializing-a-timing-function). It was converting function keywords to `cubic-bezier` or `steps` functions directly. But we needed to store it to use in the serialization step.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14822 (github issue number if applicable).

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