Add support for Low Priorirty tasks.

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();
     }
 
+    size_t allQueuesSize() const
+    {
+        return m_queue[0].size() + m_queue[1].size();
+    }
+
     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