cppgc-js: Fix TraceTrait for TracedReference

The trait is expected to return a nullptr for the base address. This
is required for ephemeron tracing to trigger eagerly tracing a value.

This will be required when Blink uses a type alias to TracedReference.

Bug: v8:12165
Change-Id: Ibe142eaff41616c9de6ae0db9878f8489a5e4142
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3253345Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77619}

include/v8-cppgc.h

@@ -318,6 +318,11 @@ namespace cppgc {
 
 template <typename T>
 struct TraceTrait<v8::TracedReference<T>> {
+  static cppgc::TraceDescriptor GetTraceDescriptor(
+      const v8::TracedReference<T>* ref) {
+    return {nullptr, Trace};
+  }
+
   static void Trace(Visitor* visitor, const v8::TracedReference<T>* self) {
     static_cast<v8::JSVisitor*>(visitor)->Trace(*self);
   }