Minor refactoring of the parsing of keyframes.

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- [X] These changes fix  #20179 (github issue number if applicable).

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- [X] These changes do not require tests because it is a minor refactor so tests are already in place. The original Issue explicitly says so.

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Source-Repo: https://github.com/servo/servo
Source-Revision: 29e10d4f88a7564f4d6a4496d3d1be5949962f8a

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : cf1ac3eb68cdf37d44f5c1e0a9e478f2a23bc3d8