Clean up forward declarations in Source/platform

Source/platform/heap/Heap.h

 /*
  * 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 102 matching lines @@
 enum CallbackInvocationMode {
     GlobalMarking,
     ThreadLocalMarking,
 };
 
 class CallbackStack;
 class HeapStats;
 class PageMemory;
 template<ThreadAffinity affinity> class ThreadLocalPersistents;
 template<typename T, typename RootsAccessor = ThreadLocalPersistents<ThreadingTrait<T>::Affinity > > class Persistent;
-template<typename T> class CrossThreadPersistent;
 
 #if ENABLE(GC_PROFILE_HEAP)
 class TracedValue;
 #endif
 
 PLATFORM_EXPORT size_t osPageSize();
 
 // Blink heap pages are set up with a guard page before and after the
 // payload.
 inline size_t blinkPagePayloadSize()
@@ skipping 2265 matching lines @@
 };
 
 template<typename T>
 struct IfWeakMember<WeakMember<T> > {
     static bool isDead(Visitor* visitor, const WeakMember<T>& t) { return !visitor->isAlive(t.get()); }
 };
 
 }
 
 #endif // Heap_h