[oilpan] Resurrect a loop-back persistent handle in RefCountedHeapAllocated

Source/wtf/SizeLimits.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 25 matching lines @@
 #include <wtf/RefCounted.h>
 #include <wtf/RefPtr.h>
 #include <wtf/ThreadRestrictionVerifier.h>
 #include <wtf/Vector.h>
 
 namespace WTF {
 
 #ifndef NDEBUG
 struct SameSizeAsRefCounted {
     int a;
-    bool b;
-    bool c;
-    ThreadRestrictionVerifier d;
+    // FIXME(oilpan): Remove the following comment-outs once we replace
+    // all RefCountedHeapAllocated with HeapAllocated.
+    // bool b;
+    // bool c;
+    // ThreadRestrictionVerifier d;
     // The debug version may get bigger.
 };
 #else
 struct SameSizeAsRefCounted {
     int a;
     // Don't add anything here because this should stay small.
 };
 #endif
 
 COMPILE_ASSERT(sizeof(OwnPtr<int>) == sizeof(int*), OwnPtr_should_stay_small);
 COMPILE_ASSERT(sizeof(PassRefPtr<RefCounted<int> >) == sizeof(int*), PassRefPtr_should_stay_small);
 COMPILE_ASSERT(sizeof(RefCounted<int>) == sizeof(SameSizeAsRefCounted), RefCounted_should_stay_small);
 COMPILE_ASSERT(sizeof(RefCountedCustomAllocated<int>) == sizeof(SameSizeAsRefCounted), RefCountedCustomAllocated_should_stay_small);
 COMPILE_ASSERT(sizeof(RefPtr<RefCounted<int> >) == sizeof(int*), RefPtr_should_stay_small);
 #if !(OS(WINDOWS) && CPU(X86_64))  // Packing trouble on Win64. See crbug.com/231671
 COMPILE_ASSERT(sizeof(Vector<int>) == sizeof(int*) + 2 * sizeof(int), Vector_should_stay_small);
 COMPILE_ASSERT(sizeof(Vector<int, 1>) == 2 * sizeof(int*) + 2 * sizeof(int), Vector_should_stay_small);
 #endif
 
 }