Add missing buffer reorg on Deque buffer expansion.

Source/wtf/Deque.h

 /*
  * Copyright (C) 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 313 matching lines @@
 
         expandCapacity();
     }
 
     template<typename T, size_t inlineCapacity, typename Allocator>
     void Deque<T, inlineCapacity, Allocator>::expandCapacity()
     {
         size_t oldCapacity = m_buffer.capacity();
         T* oldBuffer = m_buffer.buffer();
         size_t newCapacity = std::max(static_cast<size_t>(16), oldCapacity + oldCapacity / 4 + 1);
-        if (m_buffer.expandBuffer(newCapacity))
+        if (m_buffer.expandBuffer(newCapacity)) {
+            if (m_start <= m_end) {
+                // Expanded the buffer beyond m_end; no need to move elements
+                // nor adjust start index, everything's in place.
+                return;
+            }
+            // The expanded buffer contains [0, m_end] from the "old" buffer
+            // already, so only have to move [m_start, oldCapacity] to the
+            // back of the expanded buffer.
+            size_t newStart = m_buffer.capacity() - (oldCapacity - m_start);
+            TypeOperations::moveOverlapping(oldBuffer + m_start, oldBuffer + oldCapacity, m_buffer.buffer() + newStart);
+            m_start = newStart;
             return;
+        }
         m_buffer.allocateBuffer(newCapacity);
         if (m_start <= m_end)
             TypeOperations::move(oldBuffer + m_start, oldBuffer + m_end, m_buffer.buffer() + m_start);
         else {
             TypeOperations::move(oldBuffer, oldBuffer + m_end, m_buffer.buffer());
             size_t newStart = m_buffer.capacity() - (oldCapacity - m_start);
             TypeOperations::move(oldBuffer + m_start, oldBuffer + oldCapacity, m_buffer.buffer() + newStart);
             m_start = newStart;
         }
         m_buffer.deallocateBuffer(oldBuffer);
@@ skipping 206 matching lines @@
     struct NeedsTracing<Deque<T, N> > {
         static const bool value = false;
     };
 #endif
 
 } // namespace WTF
 
 using WTF::Deque;
 
 #endif // WTF_Deque_h