style: Use OwnedSlice in the specified and computed values of most vector properties.

This is just a refactor in the right direction. Eventual goal is:

 * All inherited properties use ArcSlice<>.
 * All reset properties use OwnedSlice<> (or ThinVec<>).

No conversion happens at all, so we can remove all that glue, and also
compute_iter and co.

Of course there's work to do, but this is a step towards that.

Differential Revision: https://phabricator.services.mozilla.com/D30127

components/style_traits/owned_slice.rs

@@ -10,7 +10,7 @@ use malloc_size_of::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
 use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
 use std::ptr::NonNull;
-use std::{fmt, mem, slice};
+use std::{fmt, iter, mem, slice};
 use to_shmem::{SharedMemoryBuilder, ToShmem};
 
 /// A struct that basically replaces a `Box<[T]>`, but which cbindgen can
@@ -164,3 +164,10 @@ impl<T: ToShmem + Sized> ToShmem for OwnedSlice<T> {
         }
     }
 }
+
+impl<T> iter::FromIterator<T> for OwnedSlice<T> {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
+        Vec::from_iter(iter).into()
+    }
+}