Show debug information for invalid 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 185 matching lines @@
         DEBUG_STRING_CASE(BeginSubsequence);
         DEBUG_STRING_CASE(EndSubsequence);
         DEBUG_STRING_CASE(CachedSubsequence);
         DEBUG_STRING_CASE(UninitializedType);
         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());
+    if (!isValid()) {
+        stringBuilder.append("valid: false, originalDebugString: ");
+        // This is the original debug string which is in json format.
+        stringBuilder.append(clientDebugString());
+        return;
+    }
+
     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