Avoid re-iterate out-of-order display items

Source/platform/graphics/paint/DisplayItem.cpp

 // 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.
 
 #include "config.h"
 #include "platform/graphics/paint/DisplayItem.h"
 
 namespace blink {
 
 struct SameSizeAsDisplayItem {
@@ skipping 172 matching lines @@
         DEBUG_STRING_CASE(BeginFixedPosition);
         DEBUG_STRING_CASE(EndFixedPosition);
         DEBUG_STRING_CASE(BeginFixedPositionContainer);
         DEBUG_STRING_CASE(EndFixedPositionContainer);
         DEFAULT_CASE;
     }
 }
 
 WTF::String DisplayItem::asDebugString() const
 {
+    if (!isValid())
+        return "null";
     WTF::StringBuilder stringBuilder;
     stringBuilder.append('{');
     dumpPropertiesAsDebugString(stringBuilder);
     stringBuilder.append('}');
     return stringBuilder.toString();
 }
 
 void DisplayItem::dumpPropertiesAsDebugString(WTF::StringBuilder& stringBuilder) const
 {
+    ASSERT(isValid());
     stringBuilder.append(String::format("client: \"%p", client()));
     if (!clientDebugString().isEmpty()) {
         stringBuilder.append(' ');
         stringBuilder.append(clientDebugString());
     }
     stringBuilder.append("\", type: \"");
     stringBuilder.append(typeAsDebugString(type()));
     stringBuilder.append('"');
     if (m_skippedCache)
         stringBuilder.append(", skippedCache: true");
     if (m_scope)
         stringBuilder.append(String::format(", scope: %d", m_scope));
 }
 
 #endif
 
 } // namespace blink