Restrict registered PersistentNodes to non-empty ones.

Source/platform/heap/PersistentNode.cpp

Index: Source/platform/heap/PersistentNode.cpp
diff --git a/Source/platform/heap/PersistentNode.cpp b/Source/platform/heap/PersistentNode.cpp
index 6271f7d6ea72c36c01868d2a242d3f18b4db3406..2e986858b4579d76d6fa38937409518a711f09c6 100644
--- a/Source/platform/heap/PersistentNode.cpp
+++ b/Source/platform/heap/PersistentNode.cpp
@@ -92,6 +92,13 @@ void PersistentRegion::tracePersistentNodes(Visitor* visitor)
     ASSERT(persistentCount == m_persistentCount);
 }
 
+namespace {
+class GCObject final : public GarbageCollected<GCObject> {
+public:
+    DEFINE_INLINE_TRACE() { }
+};
+}
+
 void CrossThreadPersistentRegion::prepareForThreadStateTermination(ThreadState* threadState)
 {
     // For heaps belonging to a thread that's detaching, any cross-thread persistents
@@ -99,9 +106,6 @@ void CrossThreadPersistentRegion::prepareForThreadStateTermination(ThreadState*
     // heap reference.
     MutexLocker lock(m_mutex);
 
-    class Object;
-    using GCObject = GarbageCollected<Object>;
-
     // TODO(sof): consider ways of reducing overhead. (e.g., tracking number of active
     // CrossThreadPersistent<>s pointing into the heaps of each ThreadState and use that
     // count to bail out early.)