style: Move size of tests to compile-time tests in the style crate

Same reasoning as the previous commit.

Differential Revision: https://phabricator.services.mozilla.com/D146104

components/style/macros.rs

@@ -128,3 +128,11 @@ macro_rules! local_name {
         $crate::values::AtomIdent(atom!($s))
     };
 }
+
+/// Asserts the size of a type at compile time.
+macro_rules! size_of_test {
+    ($t: ty, $expected_size: expr) => {
+        #[cfg(target_pointer_width = "64")]
+        const_assert_eq!(std::mem::size_of::<$t>(), $expected_size);
+    };
+}