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/values/computed/mod.rs

@@ -454,8 +454,7 @@ where
     fn to_computed_value(&self, context: &Context) -> Self::ComputedValue {
         self.iter()
             .map(|item| item.to_computed_value(context))
-            .collect::<Vec<_>>()
-            .into()
+            .collect()
     }
 
     #[inline]
@@ -463,8 +462,7 @@ where
         computed
             .iter()
             .map(T::from_computed_value)
-            .collect::<Vec<_>>()
-            .into()
+            .collect()
     }
 }