Oilpan: Reduce sizeof(Persistent) to 16 byte

Source/platform/heap/blink_heap.gypi

 #
 # Copyright (C) 2013 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 31 matching lines @@
       'Heap.cpp',
       'Heap.h',
       'HeapAllocator.cpp',
       'HeapAllocator.h',
       'InlinedGlobalMarkingVisitor.h',
       'MarkingVisitor.h',
       'MarkingVisitorImpl.h',
       'PageMemory.h',
       'PagePool.cpp',
       'PagePool.h',
+      'PersistentNode.cpp',
+      'PersistentNode.h',
       'SafePoint.cpp',
       'SafePoint.h',
       'StackFrameDepth.cpp',
       'StackFrameDepth.h',
       'ThreadState.cpp',
       'ThreadState.h',
       'ThreadingTraits.h',
       'TraceTraits.h',
       'Visitor.h',
     ],
@@ skipping 23 matching lines @@
         ],
       }],
       ['target_arch == "ia32" or target_arch == "x64"', {
         'platform_heap_asm_files': [
           'asm/SaveRegisters_x86.asm',
         ],
       }],
     ],
   },
 }