Implement :valid and :invalid pseudoclass for <fieldset>

Source/core/html/HTMLFieldSetElement.h

Index: Source/core/html/HTMLFieldSetElement.h
diff --git a/Source/core/html/HTMLFieldSetElement.h b/Source/core/html/HTMLFieldSetElement.h
index dd2bceb119628de4b4123cf1d329543c84c479a4..047e19e03c61c304daa3cb20576aa727fb9c4065 100644
--- a/Source/core/html/HTMLFieldSetElement.h
+++ b/Source/core/html/HTMLFieldSetElement.h
@@ -42,6 +42,8 @@ public:
 
     const FormAssociatedElement::List& associatedElements() const;
 
+    void setNeedsValidityCheck();
+
 protected:
     virtual void disabledAttributeChanged() override;
 
@@ -53,6 +55,8 @@ private:
     virtual RenderObject* createRenderer(RenderStyle*) override;
     virtual const AtomicString& formControlType() const override;
     virtual bool recalcWillValidate() const override { return false; }
+    virtual bool matchesValidityPseudoClasses() const override final;
+    virtual bool isValidElement() override final;
     virtual void childrenChanged(const ChildrenChange&) override;
     virtual bool areAuthorShadowsAllowed() const override { return false; }