Auto merge of #18462 - mrobinson:cleanup-building-state, r=emilio

Do some minor cleanups in display list building

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- [ ] These changes fix #__ (github issue number if applicable).

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components/layout/multicol.rs

@@ -10,7 +10,7 @@ use ServoArc;
 use app_units::Au;
 use block::BlockFlow;
 use context::LayoutContext;
-use display_list_builder::DisplayListBuildState;
+use display_list_builder::{DisplayListBuildState, StackingContextCollectionState};
 use euclid::{Point2D, Vector2D};
 use floats::FloatKind;
 use flow::{Flow, FlowClass, OpaqueFlow, mut_base, FragmentationContext};
@@ -190,7 +190,7 @@ impl Flow for MulticolFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, state: &mut DisplayListBuildState) {
+    fn collect_stacking_contexts(&mut self, state: &mut StackingContextCollectionState) {
         self.block_flow.collect_stacking_contexts(state);
     }
 
@@ -280,7 +280,7 @@ impl Flow for MulticolColumnFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, state: &mut DisplayListBuildState) {
+    fn collect_stacking_contexts(&mut self, state: &mut StackingContextCollectionState) {
         self.block_flow.collect_stacking_contexts(state);
     }