Implement :valid and :invalid pseudoclass for <form>

Source/core/html/HTMLFormElement.cpp

Index: Source/core/html/HTMLFormElement.cpp
diff --git a/Source/core/html/HTMLFormElement.cpp b/Source/core/html/HTMLFormElement.cpp
index 013dfd394be8dbef75ae85657bcc183e80b7aa84..e8baabf731e07ed0a3858ede534f1606dfb7755c 100644
--- a/Source/core/html/HTMLFormElement.cpp
+++ b/Source/core/html/HTMLFormElement.cpp
@@ -109,6 +109,16 @@ void HTMLFormElement::trace(Visitor* visitor)
     HTMLElement::trace(visitor);
 }
 
+bool HTMLFormElement::willValidate() const
+{
+    return true;
+}
+
+bool HTMLFormElement::isValidElement()
+{
+    return checkValidity();
+}
+
 bool HTMLFormElement::rendererIsNeeded(const RenderStyle& style)
 {
     if (!m_wasDemoted)