Invalidate :default pseudo classes when a default button is added or removed.

third_party/WebKit/Source/core/html/HTMLFormElement.h

Index: third_party/WebKit/Source/core/html/HTMLFormElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLFormElement.h b/third_party/WebKit/Source/core/html/HTMLFormElement.h
index 9bb909cbde9cd2e009faa67f6d13fdf7f6b2e006..219c15e8a8be74da880722e37482610f42fabeb2 100644
--- a/third_party/WebKit/Source/core/html/HTMLFormElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLFormElement.h
@@ -117,6 +117,7 @@ public:
     const WillBeHeapVector<RawPtrWillBeMember<HTMLImageElement>>& imageElements();
 
     void anonymousNamedGetter(const AtomicString& name, RadioNodeListOrElement&);
+    void invalidateDefaultButtonStyle() const;
 
 private:
     explicit HTMLFormElement(Document&);
@@ -173,6 +174,7 @@ private:
     bool m_associatedElementsAreDirty : 1;
     bool m_imageElementsAreDirty : 1;
     bool m_hasElementsAssociatedByParser : 1;
+    bool m_hasElementsAssociatedByFormAttribute : 1;
     bool m_didFinishParsingChildren : 1;
 
     bool m_wasUserSubmitted : 1;