Show debug information for invalid display items

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index d54146d98dbdfa01912190385d958460d06395a7..b9beb6a78024814b7aff157b3875331297332ad7 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -600,14 +600,12 @@ WTF::String DisplayItemList::displayItemsAsDebugString(const DisplayItems& list)
         const DisplayItem& displayItem = *it;
         if (i)
             stringBuilder.append(",\n");
-        if (!displayItem.isValid()) {
-            stringBuilder.append("null");
-            continue;
-        }
         stringBuilder.append(String::format("{index: %d, ", (int)i));
         displayItem.dumpPropertiesAsDebugString(stringBuilder);
-        stringBuilder.append(", cacheIsValid: ");
-        stringBuilder.append(clientCacheIsValid(displayItem.client()) ? "true" : "false");
+        if (displayItem.isValid()) {
+            stringBuilder.append(", cacheIsValid: ");
+            stringBuilder.append(clientCacheIsValid(displayItem.client()) ? "true" : "false");
+        }
         stringBuilder.append('}');
     }
     return stringBuilder.toString();