<fieldset> should be ignored in interactive validation

Source/core/html/HTMLFormControlElement.h

Index: Source/core/html/HTMLFormControlElement.h
diff --git a/Source/core/html/HTMLFormControlElement.h b/Source/core/html/HTMLFormControlElement.h
index c9914bca5b156aad0b56b1dd033cbb89a3916e1d..d891c873e6d0b76fb1f38ce6d4b0ed44d65bddf5 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -77,6 +77,8 @@ public:
 
     virtual bool canTriggerImplicitSubmission() const { return false; }
 
+    virtual bool isSubmittableElement() { return true; }
+
     // Override in derived classes to get the encoded name=value pair for submitting.
     // Return true for a successful control (see HTML4-17.13.2).
     virtual bool appendFormData(FormDataList&, bool) override { return false; }