Auto merge of #12918 - hsinewu:12902-typedefs, r=Manishearth

12902 typedefs

<!-- Please describe your changes on the following line: -->
Using type alias instead of newtype.
Removing duplicated implementation, it's already inherited.
No more type constructors and foo.0
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12902 (github issue number if applicable).

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- [X] These changes do not require tests because it's refactoring.

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components/layout/display_list_builder.rs

@@ -520,9 +520,9 @@ impl FragmentDisplayListBuilding for Fragment {
             };
 
             // Use `background-position` to get the offset.
-            let horizontal_position = model::specified(background.background_position.0.horizontal,
+            let horizontal_position = model::specified(background.background_position.horizontal,
                                                        bounds.size.width - image_size.width);
-            let vertical_position = model::specified(background.background_position.0.vertical,
+            let vertical_position = model::specified(background.background_position.vertical,
                                                      bounds.size.height - image_size.height);
 
             let abs_x = border.left + virtual_origin_x + horizontal_position + origin_x;