Add a to_js method to the casting trait code in CodegenRust.py

Replace the manual checks and calls to transmute_copy in /layout/wrapper.rs with calls to to_js

Fixes #3616

components/script/dom/bindings/codegen/CodegenRust.py

@@ -5480,6 +5480,17 @@ class GlobalGenRoots():
     }
   }
 
+  #[inline(always)]
+  #[allow(unrooted_must_root)]
+  fn to_js<T: ${toBound}+Reflectable>(base: &JS<T>) -> Option<JS<Self>> {
+    unsafe {
+        match (*base.unsafe_get()).${checkFn}() {
+            true =>  Some(base.transmute_copy()),
+            false => None
+        }
+    }
+  }
+
   #[inline(always)]
   fn from_ref<'a, T: ${fromBound}+Reflectable>(derived: JSRef<'a, T>) -> JSRef<'a, Self> {
     unsafe { derived.transmute() }