Auto merge of #13417 - mrobinson:simplify-stacking-context-collection, r=glennw

Simplify stacking context collection

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

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Simplify the way that stacking contexts are collected. Instead of
passing the StackingContextId down the tree, pass the parent
StackingContext itself. This will allow future patches to get more
information about the parent stacking context (such as location).

Also remove the return value of collect_stacking_contexts, which was
unused.

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components/layout/inline.rs

@@ -20,7 +20,6 @@ use fragment::SpecificFragmentInfo;
 use gfx::display_list::{OpaqueNode, StackingContext};
 use gfx::font::FontMetrics;
 use gfx::font_context::FontContext;
-use gfx_traits::StackingContextId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::IntrinsicISizesContribution;
@@ -1660,11 +1659,8 @@ impl Flow for InlineFlow {
 
     fn update_late_computed_block_position_if_necessary(&mut self, _: Au) {}
 
-    fn collect_stacking_contexts(&mut self,
-                                 parent_id: StackingContextId,
-                                 contexts: &mut Vec<Box<StackingContext>>)
-                                 -> StackingContextId {
-        self.collect_stacking_contexts_for_inline(parent_id, contexts)
+    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+        self.collect_stacking_contexts_for_inline(parent);
     }
 
     fn build_display_list(&mut self, state: &mut DisplayListBuildState) {