This is part of #16982.
Right now we have some code that tries to cache stuff in the style data during
layout.
This code is extremely rarely executed, only for `<details>` and `<summary>`.
I think if we really really want to cache these, we should find a place in the
layout data for it.
For now, let's move it away (and all the other layout code) from mutating the
style data from layout.