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/flow.rs

@@ -224,10 +224,7 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
         None
     }
 
-    fn collect_stacking_contexts(&mut self,
-                                 _parent_id: StackingContextId,
-                                 _: &mut Vec<Box<StackingContext>>)
-                                 -> StackingContextId;
+    fn collect_stacking_contexts(&mut self, _parent: &mut StackingContext);
 
     /// If this is a float, places it. The default implementation does nothing.
     fn place_float_if_applicable<'a>(&mut self) {}
@@ -1160,11 +1157,9 @@ impl BaseFlow {
         return self as *const BaseFlow as usize;
     }
 
-    pub fn collect_stacking_contexts_for_children(&mut self,
-                                                  parent_id: StackingContextId,
-                                                  contexts: &mut Vec<Box<StackingContext>>) {
+    pub fn collect_stacking_contexts_for_children(&mut self, parent: &mut StackingContext) {
         for kid in self.children.iter_mut() {
-            kid.collect_stacking_contexts(parent_id, contexts);
+            kid.collect_stacking_contexts(parent);
         }
     }