Add missing buffer reorg on Deque buffer expansion.

Source/wtf/Deque.h

Index: Source/wtf/Deque.h
diff --git a/Source/wtf/Deque.h b/Source/wtf/Deque.h
index b0ab0c4260a20d6ab0759419908300b08f628d54..a838a06b17143d08762af6500186265d137b8b5b 100644
--- a/Source/wtf/Deque.h
+++ b/Source/wtf/Deque.h
@@ -331,8 +331,20 @@ namespace WTF {
         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);