Show debug information for invalid 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 25b0bcf79e4bb6e243f672e6e88edb6c865b86af..3f114622cffa72ea9f2c2339a89ad57ffbb3721d 100644
--- a/Source/platform/graphics/paint/DisplayItem.cpp
+++ b/Source/platform/graphics/paint/DisplayItem.cpp
@@ -203,8 +203,6 @@ WTF::String DisplayItem::typeAsDebugString(Type type)
 
 WTF::String DisplayItem::asDebugString() const
 {
-    if (!isValid())
-        return "null";
     WTF::StringBuilder stringBuilder;
     stringBuilder.append('{');
     dumpPropertiesAsDebugString(stringBuilder);
@@ -214,7 +212,12 @@ WTF::String DisplayItem::asDebugString() const
 
 void DisplayItem::dumpPropertiesAsDebugString(WTF::StringBuilder& stringBuilder) const
 {
-    ASSERT(isValid());
+    if (!isValid()) {
+        stringBuilder.append("valid: false, originalDebugString: ");
+        stringBuilder.append(clientDebugString());

[pdr., 2015/09/03 01:59:20]

Can you add quotes around this so it's valid-ish json?

[Xianzhu, 2015/09/03 05:48:44]

It's the original display item's debug string which is already in json format.
Will add a comment here.

+        return;
+    }
+
     stringBuilder.append(String::format("client: \"%p", client()));
     if (!clientDebugString().isEmpty()) {
         stringBuilder.append(' ');