StackingContexts are added to layers when it is necessary to maintain their ordering on top of other layered StackingContexts. Instead of tracking the information about a layer scattered around into different structs, combine it all into LayerInfo. LayerInfo will be used in the future to hold layer information for DisplayItems that are layerized independently of StackingContexts. Source-Repo: https://github.com/servo/servo Source-Revision: 27d468b6674b81c396aa0a13d8fe9eb410c12d6e