Auto merge of #29677 - mrobinson:resolved-insets, r=delan

Better implement getComputedStyle() for positioned insets

The specification dictates quite quite idiosyncratic return values when
querying insets of positioned elements via `getComputedStyle()`. These
depend on whether or not the element's size was overconstrained. This
change adds a better implementation of that in preparation for returning
proper values for `position: sticky`.

There are two changes here:
- The containing block used in `FragmentTree::find` is properly computed using the `ContainingBlockManager` data structure. This data structure is now updated during traversal through the fragment tree in `Fragment::find`.
- We now take into account whether or not a absolute elements are overconstrained when calculating their resolved insets. For absolutely positioned elements, this happens during absolute positioning. For relatively positioned elements, we can determine this when a `BoxFragment` is constructed.

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