Implement Custom Elements 'extends' option.

Source/core/dom/CustomElementRegistry.cpp

Index: Source/core/dom/CustomElementRegistry.cpp
diff --git a/Source/core/dom/CustomElementRegistry.cpp b/Source/core/dom/CustomElementRegistry.cpp
index b734c82a94060ef722df0b1be55276ad6768be78..00d6d71b81a83c162260942c46d270913c9cd859 100644
--- a/Source/core/dom/CustomElementRegistry.cpp
+++ b/Source/core/dom/CustomElementRegistry.cpp
@@ -78,16 +78,13 @@ CustomElementDefinition* CustomElementRegistry::registerElement(Document* docume
         return 0;
     }
 
-    if (!constructorBuilder->validateOptions(type, es))
-        return 0;
-
     QualifiedName tagName = nullQName();
-    if (!constructorBuilder->findTagName(type, tagName)) {
-        CustomElementException::throwException(CustomElementException::PrototypeDoesNotExtendHTMLElementSVGElementNamespace, type, es);
+    if (!constructorBuilder->validateOptions(type, tagName, es))
         return 0;
-    }
+
     ASSERT(tagName.namespaceURI() == HTMLNames::xhtmlNamespaceURI || tagName.namespaceURI() == SVGNames::svgNamespaceURI);
 
+    // FIXME: This should be done earlier in validateOptions.
     if (m_registeredTypeNames.contains(type)) {
         CustomElementException::throwException(CustomElementException::TypeAlreadyRegistered, type, es);
         return 0;