style: Use the owned slice type for basic shape polygon coordinates.

This enables destructors for tagged unions in cbindgen, implemented in:

 * https://github.com/eqrion/cbindgen/pull/333

Which allow us to properly generate a destructor for the cbindgen-generated
StyleBasicShape (which now contains an OwnedSlice).

For now, we still use the glue code to go from Box<BasicShape> to
UniquePtr<BasicShape>. But that will change in the future when we generate even
more stuff and remove all the glue.

I could add support for copy-constructor generation to cbindgen for tagged
enums, but I'm not sure if it'll end up being needed, and copy-constructing
unions in C++ is always very tricky.

Differential Revision: https://phabricator.services.mozilla.com/D29769

components/style/values/animated/mod.rs

@@ -16,7 +16,6 @@ use crate::values::computed::Image;
 use crate::values::specified::SVGPathData;
 use crate::values::CSSFloat;
 use app_units::Au;
-use euclid::Point2D;
 use smallvec::SmallVec;
 use std::cmp;
 
@@ -241,16 +240,10 @@ impl Animate for Au {
     }
 }
 
-impl<T> Animate for Point2D<T>
-where
-    T: Animate,
-{
+impl<T: Animate> Animate for Box<T> {
     #[inline]
     fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
-        Ok(Point2D::new(
-            self.x.animate(&other.x, procedure)?,
-            self.y.animate(&other.y, procedure)?,
-        ))
+        Ok(Box::new((**self).animate(&other, procedure)?))
     }
 }
 
@@ -288,6 +281,24 @@ where
     }
 }
 
+impl<T> ToAnimatedValue for Box<T>
+where
+    T: ToAnimatedValue,
+{
+    type AnimatedValue = Box<<T as ToAnimatedValue>::AnimatedValue>;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        Box::new((*self).to_animated_value())
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        Box::new(T::from_animated_value(*animated))
+    }
+}
+
+
 impl<T> ToAnimatedValue for Box<[T]>
 where
     T: ToAnimatedValue,
@@ -328,7 +339,7 @@ where
 
     #[inline]
     fn from_animated_value(animated: Self::AnimatedValue) -> Self {
-        Box::from_animated_value(animated.into_box()).into()
+        Self::from(Box::from_animated_value(animated.into_box()))
     }
 }