bindings: Move string-related bindings code to script_bindings. (#35172)

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/trace.rs

@@ -0,0 +1,24 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::str::{DOMString, USVString};
+
+/// For use on non-jsmanaged types
+/// Use #[derive(JSTraceable)] on JS managed types
+macro_rules! unsafe_no_jsmanaged_fields(
+    ($($ty:ty),+) => (
+        $(
+            #[allow(unsafe_code)]
+            unsafe impl crate::JSTraceable for $ty {
+                #[inline]
+                unsafe fn trace(&self, _: *mut ::js::jsapi::JSTracer) {
+                    // Do nothing
+                }
+            }
+        )+
+    );
+);
+
+unsafe_no_jsmanaged_fields!(DOMString);
+unsafe_no_jsmanaged_fields!(USVString);