Remove WTF::DoubleBufferedDeque.

Source/wtf/DoubleBufferedDeque.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 DoubleBufferedDeque_h
-#define DoubleBufferedDeque_h
-
-#include "wtf/Deque.h"
-#include "wtf/Noncopyable.h"
-
-namespace WTF {
-
-// A helper class for managing double buffered deques, typically where the client locks when appending or swapping.
-template <typename T> class DoubleBufferedDeque {
-    WTF_MAKE_NONCOPYABLE(DoubleBufferedDeque);
-public:
-    DoubleBufferedDeque()
-        : m_activeIndex(0) { }
-
-    void append(const T& value)
-    {
-        m_queue[m_activeIndex].append(value);
-    }
-
-    bool isEmpty() const
-    {
-        return m_queue[m_activeIndex].isEmpty();
-    }
-
-    Deque<T>& swapBuffers()
-    {
-        int oldIndex = m_activeIndex;
-        m_activeIndex ^= 1;
-        ASSERT(m_queue[m_activeIndex].isEmpty());
-        return m_queue[oldIndex];
-    }
-
-private:
-    Deque<T> m_queue[2];
-    int m_activeIndex;
-};
-
-} // namespace WTF
-
-using WTF::DoubleBufferedDeque;
-
-#endif // DoubleBufferedDeque_h