CSS4: Implement :placeholder-shown pseudo class

Source/core/html/HTMLTextAreaElement.h

Index: Source/core/html/HTMLTextAreaElement.h
diff --git a/Source/core/html/HTMLTextAreaElement.h b/Source/core/html/HTMLTextAreaElement.h
index 872b4bc73e7e15df085a41bf3be5ba4ba48f96ce..e45127ed0b91efb2bed93ae953c7bad3905ea98a 100644
--- a/Source/core/html/HTMLTextAreaElement.h
+++ b/Source/core/html/HTMLTextAreaElement.h
@@ -86,6 +86,8 @@ private:
     void setNonDirtyValue(const String&);
     void setValueCommon(const String&, TextFieldEventBehavior, SetValueCommonOption = NotSetSelection);
 
+    bool isPlaceholderVisible() const override { return m_isPlaceholderVisible; }
+    void setPlaceholderVisibility(bool) override;
     bool supportsPlaceholder() const override { return true; }
     void updatePlaceholderText() override;
     bool isEmptyValue() const override { return value().isEmpty(); }
@@ -142,6 +144,7 @@ private:
     mutable String m_value;
     mutable bool m_isDirty;
     bool m_valueIsUpToDate;
+    unsigned m_isPlaceholderVisible : 1;
     String m_suggestedValue;
 };