Make |extends| option in custom element behave correctly

Source/bindings/core/v8/CustomElementConstructorBuilder.cpp

Index: Source/bindings/core/v8/CustomElementConstructorBuilder.cpp
diff --git a/Source/bindings/core/v8/CustomElementConstructorBuilder.cpp b/Source/bindings/core/v8/CustomElementConstructorBuilder.cpp
index 20000fd66d7ee17ea46f1e28e3e692a0f515bdc1..9dac18b0611a8adaa0b24a6021f036e9b2e0a704 100644
--- a/Source/bindings/core/v8/CustomElementConstructorBuilder.cpp
+++ b/Source/bindings/core/v8/CustomElementConstructorBuilder.cpp
@@ -95,7 +95,7 @@ bool CustomElementConstructorBuilder::validateOptions(const AtomicString& type,
     }
 
     AtomicString extends;
-    bool extendsProvidedAndNonNull = DictionaryHelper::get(*m_options, "extends", extends);
+    bool extendsProvidedAndNonNull = DictionaryHelper::get(*m_options, "extends", extends) && extends != "null";
 
     if (tryCatch.HasCaught()) {
         tryCatch.ReThrow();