Creating customized built-in elements without relevant definitions registered first shouldn't throw NotFoundError;

per spec change: https://github.com/w3c/webcomponents/issues/608

Tag UXP Issue #1344

dom/base/nsDocument.h

@@ -1388,20 +1388,6 @@ protected:
 private:
   static bool CustomElementConstructor(JSContext* aCx, unsigned aArgc, JS::Value* aVp);
 
-  /**
-   * Check if the passed custom element name, aOptions.mIs, is a registered
-   * custom element type or not, then return the custom element name for future
-   * usage.
-   *
-   * If there is no existing custom element definition for this name, throw a
-   * NotFoundError.
-   */
-  const nsString* CheckCustomElementName(
-    const mozilla::dom::ElementCreationOptions& aOptions,
-    const nsAString& aLocalName,
-    uint32_t aNamespaceID,
-    ErrorResult& rv);
-
 public:
   virtual already_AddRefed<mozilla::dom::CustomElementRegistry>
     GetCustomElementRegistry() override;