style: Use rust lengths for border corners.

The test in https://github.com/web-platform-tests/wpt/pull/15423 hasn't been
synced over yet, but it passes with this patch of course.

Differential Revision: https://phabricator.services.mozilla.com/D20960

components/style/values/computed/border.rs

@@ -87,6 +87,16 @@ impl BorderCornerRadius {
 }
 
 impl BorderRadius {
+    /// Returns a `0` border radius.
+    pub fn zero() -> Self {
+        Self {
+            top_left: BorderCornerRadius::zero(),
+            top_right: BorderCornerRadius::zero(),
+            bottom_right: BorderCornerRadius::zero(),
+            bottom_left: BorderCornerRadius::zero(),
+        }
+    }
+
     /// Returns whether all the values are `0px`.
     pub fn all_zero(&self) -> bool {
         fn all(corner: &BorderCornerRadius) -> bool {

components/style/values/generics/border.rs

@@ -53,7 +53,10 @@ pub use self::GenericBorderImageSlice as BorderImageSlice;
     ToComputedValue,
     ToCss,
 )]
-pub struct BorderCornerRadius<L>(#[css(field_bound)] pub Size2D<L>);
+#[repr(C)]
+pub struct GenericBorderCornerRadius<L>(#[css(field_bound)] pub Size2D<L>);
+
+pub use self::GenericBorderCornerRadius as BorderCornerRadius;
 
 impl<L> BorderCornerRadius<L> {
     /// Trivially create a `BorderCornerRadius`.
@@ -77,6 +80,7 @@ impl<L> BorderCornerRadius<L> {
     ToComputedValue,
     ToCss,
 )]
+#[repr(transparent)]
 pub struct BorderSpacing<L>(#[css(field_bound)] pub Size2D<L>);
 
 impl<L> BorderSpacing<L> {
@@ -101,17 +105,20 @@ impl<L> BorderSpacing<L> {
     ToAnimatedValue,
     ToComputedValue,
 )]
-pub struct BorderRadius<LengthPercentage> {
+#[repr(C)]
+pub struct GenericBorderRadius<LengthPercentage> {
     /// The top left radius.
-    pub top_left: BorderCornerRadius<LengthPercentage>,
+    pub top_left: GenericBorderCornerRadius<LengthPercentage>,
     /// The top right radius.
-    pub top_right: BorderCornerRadius<LengthPercentage>,
+    pub top_right: GenericBorderCornerRadius<LengthPercentage>,
     /// The bottom right radius.
-    pub bottom_right: BorderCornerRadius<LengthPercentage>,
+    pub bottom_right: GenericBorderCornerRadius<LengthPercentage>,
     /// The bottom left radius.
-    pub bottom_left: BorderCornerRadius<LengthPercentage>,
+    pub bottom_left: GenericBorderCornerRadius<LengthPercentage>,
 }
 
+pub use self::GenericBorderRadius as BorderRadius;
+
 impl<L> BorderRadius<L> {
     /// Returns a new `BorderRadius<L>`.
     #[inline]

components/style/values/generics/size.rs

@@ -25,6 +25,7 @@ use style_traits::{CssWriter, ParseError, ToCss};
     ToComputedValue,
 )]
 #[allow(missing_docs)]
+#[repr(C)]
 pub struct Size2D<L> {
     pub width: L,
     pub height: L,