stylo: Use Arc::get_mut().unwrap() instead of make_mut.

It's infallible, so no point in using make_mut.

components/style/properties/gecko.mako.rs

@@ -425,7 +425,7 @@ impl ${style_struct.gecko_struct_name} {
     pub fn initial() -> Arc<Self> {
         let mut result = Arc::new(${style_struct.gecko_struct_name} { gecko: unsafe { zeroed() } });
         unsafe {
-            Gecko_Construct_${style_struct.gecko_ffi_name}(&mut Arc::make_mut(&mut result).gecko);
+            Gecko_Construct_${style_struct.gecko_ffi_name}(&mut Arc::get_mut(&mut result).unwrap().gecko);
         }
         result
     }