OwnPtr, OwnArrayPtr: Use copy/move-and-swap for assignment operators

Source/wtf/OwnPtr.h

Index: Source/wtf/OwnPtr.h
diff --git a/Source/wtf/OwnPtr.h b/Source/wtf/OwnPtr.h
index e5baf9341e8ae4ff56e68536d470c6dedbfb087b..61609386abaa6b23033030dcc9adfc37c3a38c16 100644
--- a/Source/wtf/OwnPtr.h
+++ b/Source/wtf/OwnPtr.h
@@ -137,19 +137,17 @@ namespace WTF {
 
     template<typename T> inline OwnPtr<T>& OwnPtr<T>::operator=(const PassOwnPtr<T>& o)
     {
-        PtrType ptr = m_ptr;
-        m_ptr = o.leakPtr();
-        ASSERT(!ptr || m_ptr != ptr);
-        deleteOwnedPtr(ptr);
+        ASSERT(!o || o != m_ptr);
+        OwnPtr ptr = o;
+        swap(ptr);
         return *this;
     }
 
     template<typename T> template<typename U> inline OwnPtr<T>& OwnPtr<T>::operator=(const PassOwnPtr<U>& o)
     {
-        PtrType ptr = m_ptr;
-        m_ptr = o.leakPtr();
-        ASSERT(!ptr || m_ptr != ptr);
-        deleteOwnedPtr(ptr);
+        ASSERT(!o || o != m_ptr);
+        OwnPtr ptr = o;
+        swap(ptr);
         return *this;
     }