style: Part 2 - Add page-size attribute to nsStyleStruct and property parsing

This parsing is hidden behind the pref layout.css.page-size.enabled.

It isn't ideal that we parse this as a property, but we can't treat it as a
descriptor because of compatibility issues with other browsers. There are also
outstanding spec issues related to how descriptors like page-size are cascaded,
and whether the !important specifier is valid or not.

Differential Revision: https://phabricator.services.mozilla.com/D103958

components/style/values/generics/page.rs

@@ -86,7 +86,7 @@ pub enum Orientation {
     ToShmem,
 )]
 #[repr(C, u8)]
-pub enum PageSize<S> {
+pub enum GenericPageSize<S> {
     /// Page dimensions.
     Size(S),
     /// Paper size with no orientation.
@@ -99,6 +99,8 @@ pub enum PageSize<S> {
     Auto,
 }
 
+pub use self::GenericPageSize as PageSize;
+
 impl<S> PageSize<S> {
     /// `auto` value.
     #[inline]