CSS4: Implement :placeholder-shown pseudo class

Source/core/html/HTMLInputElement.h

Index: Source/core/html/HTMLInputElement.h
diff --git a/Source/core/html/HTMLInputElement.h b/Source/core/html/HTMLInputElement.h
index d39f33283f05c6c4cc009240d4377cc4eed6d50b..31b243a484a3665ee229c677a999bf6b0d3e1020 100644
--- a/Source/core/html/HTMLInputElement.h
+++ b/Source/core/html/HTMLInputElement.h
@@ -259,6 +259,10 @@ public:
     virtual void ensureFallbackContent();
     virtual void ensurePrimaryContent();
     bool hasFallbackContent() const;
+
+    bool isPlaceholderVisible() const override { return m_isPlaceholderVisible; }
+    void setPlaceholderVisibility(bool) override;
+
 protected:
     HTMLInputElement(Document&, HTMLFormElement*, bool createdByParser);
 
@@ -384,6 +388,7 @@ private:
     unsigned m_hasTouchEventHandler : 1;
     unsigned m_shouldRevealPassword : 1;
     unsigned m_needsToUpdateViewValue : 1;
+    unsigned m_isPlaceholderVisible : 1;
     RefPtrWillBeMember<InputType> m_inputType;
     RefPtrWillBeMember<InputTypeView> m_inputTypeView;
     // The ImageLoader must be owned by this element because the loader code assumes