Implement Custom Elements 'extends' option.

Source/core/dom/CustomElementException.cpp

Index: Source/core/dom/CustomElementException.cpp
diff --git a/Source/core/dom/CustomElementException.cpp b/Source/core/dom/CustomElementException.cpp
index b5752a8c02d54a0a20601b1812abca10be07c5d0..73b1717819a5b39ae5f41e8f7dddcd8968ddb1a3 100644
--- a/Source/core/dom/CustomElementException.cpp
+++ b/Source/core/dom/CustomElementException.cpp
@@ -64,20 +64,16 @@ void CustomElementException::throwException(Reason reason, const AtomicString& t
         es.throwDOMException(NotSupportedError);
         return;
 
-    case InvalidName:
-        es.throwDOMException(InvalidCharacterError, preamble(type) + ": '" + type + "' is not a valid name.");
+    case ExtendsIsInvalidName:
+        es.throwDOMException(InvalidCharacterError, preamble(type) + ": the tag name specified in 'extends' is not a valid tag name.");
         return;
 
-    case NotYetImplemented:
-        es.throwDOMException(InvalidStateError);
+    case ExtendsIsCustomElementName:
+        es.throwDOMException(InvalidCharacterError, preamble(type) + ": the tag name specified in 'extends' is a custom element name. Use inheritance instead.");
         return;
 
-    case PrototypeDoesNotExtendHTMLElementSVGElementNamespace:
-        es.throwDOMException(NamespaceError, preamble(type) + "the prototype does not extend an HTML or SVG element.");
-        return;
-
-    case PrototypeDoesNotExtendHTMLElementSVGElementPrototype:
-        es.throwDOMException(InvalidStateError, preamble(type) + "the prototype does not extend an HTML or SVG element.");
+    case InvalidName:
+        es.throwDOMException(InvalidCharacterError, preamble(type) + ": '" + type + "' is not a valid name.");
         return;
 
     case PrototypeInUse: