Oilpan: Reduce sizeof(Persistent) to 16 byte

Source/platform/heap/BUILD.gn

 # Copyright 2014 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.
 
 source_set("heap") {
   # This target is a logical part of the platform and only the platform target
   # should depend on it.
   visibility = [ "//third_party/WebKit/Source/platform" ]
 
   sources = [
     "BlinkGCMemoryDumpProvider.cpp",
     "BlinkGCMemoryDumpProvider.h",
     "CallbackStack.cpp",
     "CallbackStack.h",
     "GCInfo.cpp",
     "GCInfo.h",
     "GarbageCollected.h",
     "Handle.h",
     "Heap.cpp",
     "Heap.h",
     "HeapAllocator.cpp",
     "HeapAllocator.h",
     "InlinedGlobalMarkingVisitor.h",
     "MarkingVisitor.h",
     "MarkingVisitorImpl.h",
     "PageMemory.h",
     "PagePool.cpp",
     "PagePool.h",
+    "PersistentNode.cpp",
+    "PersistentNode.h",
     "StackFrameDepth.cpp",
     "StackFrameDepth.h",
     "SafePoint.cpp",
     "SafePoint.h",
     "ThreadState.cpp",
     "ThreadState.h",
     "ThreadingTraits.h",
     "TraceTraits.h",
     "Visitor.h",
   ]
 
   defines = [ "BLINK_PLATFORM_IMPLEMENTATION=1" ]
 
   configs += [
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 
   deps = [
     "//third_party/WebKit/Source/platform/heap/asm",
     "//third_party/icu",
   ]
 }