Bump cssom from 0.3.4 to 0.3.8 (#13461)

Bumps [cssom](https://github.com/NV/CSSOM) from 0.3.4 to 0.3.8.
- [Release notes](https://github.com/NV/CSSOM/releases)
- [Commits](https://github.com/NV/CSSOM/commits

)

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