Auto merge of #14746 - DominoTree:fix-linear-gradient, r=emilio

Default is top-to-bottom if unset, not bottom-to-top

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Reverse linear gradient direction if not explicitly specified to match expected default behavior

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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 fix #14745 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/values/computed/image.rs

@@ -470,7 +470,7 @@ impl ToComputedValue for specified::AngleOrCorner {
     fn to_computed_value(&self, _: &Context) -> AngleOrCorner {
         match *self {
             specified::AngleOrCorner::None => {
-                AngleOrCorner::Angle(Angle(0.0))
+                AngleOrCorner::Angle(Angle(PI))
             },
             specified::AngleOrCorner::Angle(angle) => {
                 AngleOrCorner::Angle(angle)