Auto merge of #16800 - Manishearth:border-radius, r=emilio

Allow border radii to be set via preshints

Fixes rendering of `<hr>`

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components/style/values/generics/mod.rs

@@ -26,6 +26,12 @@ impl<L> HasViewportPercentage for BorderRadiusSize<L> {
     fn has_viewport_percentage(&self) -> bool { false }
 }
 
+impl<L: Clone> From<L> for BorderRadiusSize<L> {
+    fn from(other: L) -> Self {
+        Self::new(other.clone(), other)
+    }
+}
+
 impl<L> BorderRadiusSize<L> {
     #[inline]
     /// Create a new `BorderRadiusSize` for an area of given width and height.