Avoid re-iterate out-of-order display items

Source/platform/graphics/paint/DisplayItem.cpp

Index: Source/platform/graphics/paint/DisplayItem.cpp
diff --git a/Source/platform/graphics/paint/DisplayItem.cpp b/Source/platform/graphics/paint/DisplayItem.cpp
index 81c92baa688fbad61cf60495dca338bf239d078d..315147e50412147b37c3d6cc029a6b04b2323b49 100644
--- a/Source/platform/graphics/paint/DisplayItem.cpp
+++ b/Source/platform/graphics/paint/DisplayItem.cpp
@@ -190,6 +190,8 @@ WTF::String DisplayItem::typeAsDebugString(Type type)
 
 WTF::String DisplayItem::asDebugString() const
 {
+    if (!isValid())
+        return "null";
     WTF::StringBuilder stringBuilder;
     stringBuilder.append('{');
     dumpPropertiesAsDebugString(stringBuilder);
@@ -199,6 +201,7 @@ WTF::String DisplayItem::asDebugString() const
 
 void DisplayItem::dumpPropertiesAsDebugString(WTF::StringBuilder& stringBuilder) const
 {
+    ASSERT(isValid());
     stringBuilder.append(String::format("client: \"%p", client()));
     if (!clientDebugString().isEmpty()) {
         stringBuilder.append(' ');