Auto merge of #29740 - mrobinson:refactor-2020-layout-for-scroll-tree, r=mukilan

Refactor Layout 2020 layout for a compositor-side scroll tree

This change refactors how layout is done in Layout 2020 in preparation for a compositor-side scroll tree:

1. Now the SpatialId and ClipId of each fragment is stored separately. This will allow storing a scroll node id instead of only the handle to the WebRender spatial node.
2. Separate out stacking context tree construction and display list building. This change will make it possible to eventually build the stacking context tree without the full display list if we find that necessary. For instance, this might be useful to cache containing block boundaries.
3. Add a `DisplayList` struct that stores both the WebRender display list builder and the compositor info. This exposes the API to the layout thread for display list building.

In addition, this change adds a lot of missing documentation.  This should not change behavior.

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- [x] These changes do not require tests because they do not change behavior.

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