Restrict registered PersistentNodes to non-empty ones.

Source/platform/heap/PersistentNode.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "platform/heap/PersistentNode.h"
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
@@ skipping 74 matching lines @@
                 freeListLast->setFreeListNext(m_freeListHead);
                 m_freeListHead = freeListNext;
             }
             prevNext = &slots->m_next;
             slots = slots->m_next;
         }
     }
     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
     // pointing into them needs to be disabled. Do that by clearing out the underlying
     // 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.)
     PersistentNodeSlots* slots = m_persistentRegion->m_slots;
     while (slots) {
         for (int i = 0; i < PersistentNodeSlots::slotCount; ++i) {
             if (slots->m_slot[i].isUnused())
                 continue;
 
             // 'self' is in use, containing the cross-thread persistent wrapper object.
             CrossThreadPersistent<GCObject>* persistent = reinterpret_cast<CrossThreadPersistent<GCObject>*>(slots->m_slot[i].self());
             ASSERT(persistent);
             void* rawObject = persistent->get();
             if (!rawObject)
                 continue;
             BasePage* page = pageFromObject(rawObject);
             ASSERT(page);
             // The main thread will upon detach just mark its heap pages as orphaned,
             // but not invalidate its CrossThreadPersistent<>s.
             if (page->orphaned())
                 continue;
             if (page->heap()->threadState() == threadState)
                 persistent->clear();
         }
         slots = slots->m_next;
     }
 }
 
 } // namespace blink