cppgc: Remove ephemeron filter

Previously, ephemerons without a base_object_payload have been
filtered.  base_object_payload is currently used to differentiate
between GarbageCollected and just traceable objects, so we need to
pass on the empty descriptor.

Bug: chromium:1056170
Change-Id: I9cba53295779ec74dce2822b7bf83f477bc3241f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2700039Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72820}

include/cppgc/visitor.h

@@ -169,7 +169,10 @@ class V8_EXPORT Visitor {
     const K* k = key.GetRawAtomic();
     if (!k) return;
     TraceDescriptor value_desc = TraceTrait<V>::GetTraceDescriptor(value);
-    if (!value_desc.base_object_payload) return;
+    // `value` must always be non-null. `value_desc.base_object_payload` may be
+    // null in the case that value is not a garbage-collected object but only
+    // traceable.
+    CPPGC_DCHECK(value);
     VisitEphemeron(key, value, value_desc);
   }