style: Use Serde for Transform.

Though this may make us use more space when serializing
StyleTransform, but we don't have to do extra conversion on the compostior
side, and this makes us easier to maintain the Rust type.

Differential Revision: https://phabricator.services.mozilla.com/D60045

components/style_traits/owned_slice.rs

@@ -7,6 +7,8 @@
 //! A replacement for `Box<[T]>` that cbindgen can understand.
 
 use malloc_size_of::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
+use serde::de::{Deserialize, Deserializer};
+use serde::ser::{Serialize, Serializer};
 use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
 use std::ptr::NonNull;
@@ -171,3 +173,22 @@ impl<T> iter::FromIterator<T> for OwnedSlice<T> {
         Vec::from_iter(iter).into()
     }
 }
+
+impl<T: Serialize> Serialize for OwnedSlice<T> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.deref().serialize(serializer)
+    }
+}
+
+impl<'de, T: Deserialize<'de>> Deserialize<'de> for OwnedSlice<T> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let r = Box::<[T]>::deserialize(deserializer)?;
+        Ok(r.into())
+    }
+}