Auto merge of #12831 - notriddle:master, r=Manishearth

Add a flag to dump the computed style values

I used this to trace #11818 to a style bug, rather than a layout bug.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not fix any issues
- [X] These changes do not require tests because debugging

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components/style/dom.rs

@@ -81,6 +81,8 @@ pub trait TNode : Sized + Copy + Clone {
 
     fn dump(self);
 
+    fn dump_style(self);
+
     fn traverse_preorder(self) -> TreeIterator<Self> {
         TreeIterator::new(self)
     }