Remove unused CallbackStack::swap() method.

Source/platform/heap/CallbackStack.h

 // 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.
 
 #ifndef CallbackStack_h
 #define CallbackStack_h
 
 #include "platform/heap/ThreadState.h"
+#include "wtf/Assertions.h"
 
 namespace blink {
 
 // The CallbackStack contains all the visitor callbacks used to trace and mark
 // objects. A specific CallbackStack instance contains at most bufferSize elements.
 // If more space is needed a new CallbackStack instance is created and chained
 // together with the former instance. I.e. a logical CallbackStack can be made of
 // multiple chained CallbackStack object instances.
 class CallbackStack {
 public:
@@ skipping 23 matching lines @@
     Item* pop();
 
     bool isEmpty() const;
 
     void invokeEphemeronCallbacks(Visitor*);
 
 #if ENABLE(ASSERT)
     bool hasCallbackForObject(const void*);
 #endif
 
+private:
     static const size_t blockSize = 8192;
 
-private:
     class Block {
     public:
         explicit Block(Block* next)
             : m_limit(&(m_buffer[blockSize]))
             , m_current(&(m_buffer[0]))
             , m_next(next)
         {
             clearUnused();
         }
 
@@ skipping 14 matching lines @@
 
         size_t size() const
         {
             return blockSize - (m_limit - m_current);
         }
 
         Item* allocateEntry()
         {
             if (LIKELY(m_current < m_limit))
                 return m_current++;
-            return 0;
+            return nullptr;
         }
 
         Item* pop()
         {
             if (UNLIKELY(isEmptyBlock()))
-                return 0;
+                return nullptr;
             return --m_current;
         }
 
         void invokeEphemeronCallbacks(Visitor*);
 #if ENABLE(ASSERT)
         bool hasCallbackForObject(const void*);
 #endif
 
     private:
         void clearUnused();
 
         Item m_buffer[blockSize];
         Item* m_limit;
         Item* m_current;
         Block* m_next;
     };
 
     Item* popSlow();
     Item* allocateEntrySlow();
     void invokeOldestCallbacks(Block*, Block*, Visitor*);
     bool hasJustOneBlock() const;
-    void swap(CallbackStack* other);
 
     Block* m_first;
     Block* m_last;
 };
 
 ALWAYS_INLINE CallbackStack::Item* CallbackStack::allocateEntry()
 {
     Item* item = m_first->allocateEntry();
     if (LIKELY(!!item))
         return item;
 
     return allocateEntrySlow();
 }
 
 ALWAYS_INLINE CallbackStack::Item* CallbackStack::pop()
 {
     Item* item = m_first->pop();
     if (LIKELY(!!item))
         return item;
 
     return popSlow();
 }
 
 } // namespace blink
 
 #endif // CallbackStack_h