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/style/properties/gecko.mako.rs

@@ -1041,16 +1041,16 @@ fn static_assert() {
     pub fn clone_background_position(&self) -> longhands::background_position::computed_value::T {
         use values::computed::position::Position;
         let position = &self.gecko.mImage.mLayers.mFirstElement.mPosition;
-        longhands::background_position::computed_value::T(Position {
+        Position {
             horizontal: position.mXPosition.into(),
             vertical: position.mYPosition.into(),
-        })
+        }
     }
 
     pub fn set_background_position(&mut self, v: longhands::background_position::computed_value::T) {
         let position = &mut self.gecko.mImage.mLayers.mFirstElement.mPosition;
-        position.mXPosition = v.0.horizontal.into();
-        position.mYPosition = v.0.vertical.into();
+        position.mXPosition = v.horizontal.into();
+        position.mYPosition = v.vertical.into();
         self.gecko.mImage.mPositionXCount = 1;
         self.gecko.mImage.mPositionYCount = 1;
     }