Introducing new API placeHolder() to expose placeholder attribute on MAC

Source/web/WebAXObject.cpp

Index: Source/web/WebAXObject.cpp
diff --git a/Source/web/WebAXObject.cpp b/Source/web/WebAXObject.cpp
index 5ec50ff479bed5a8bfdb9863db987c3071ad247a..dd8fbab025c68a2f61874454fcb58084b8f7d456 100644
--- a/Source/web/WebAXObject.cpp
+++ b/Source/web/WebAXObject.cpp
@@ -237,6 +237,14 @@ WebString WebAXObject::ariaAutoComplete() const
     return m_private->ariaAutoComplete();
 }
 
+WebString WebAXObject::placeHolder() const
+{
+    if (isDetached())
+        return WebString();
+
+    return WebString(m_private->placeHolder());
+}
+
 bool WebAXObject::isButtonStateMixed() const
 {
     if (isDetached())