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

Source/modules/accessibility/AXObject.h

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 428 matching lines @@
     virtual String actionVerb() const;
     virtual AccessibilityButtonState checkboxOrRadioValue() const;
     virtual void colorValue(int& r, int& g, int& b) const { r = 0; g = 0; b = 0; }
     virtual InvalidState invalidState() const { return InvalidStateUndefined; }
     // Only used when invalidState() returns InvalidStateOther.
     virtual String ariaInvalidValue() const { return String(); }
     virtual String valueDescription() const { return String(); }
     virtual float valueForRange() const { return 0.0f; }
     virtual float maxValueForRange() const { return 0.0f; }
     virtual float minValueForRange() const { return 0.0f; }
+    virtual String placeholder() const { return String(); }
     virtual String stringValue() const { return String(); }
     virtual const AtomicString& textInputType() const { return nullAtom; }
 
     // ARIA attributes.
     virtual AXObject* activeDescendant() const { return 0; }
     virtual String ariaAutoComplete() const { return String(); }
     virtual String ariaDescribedByAttribute() const { return String(); }
     virtual void ariaFlowToElements(AccessibilityChildrenVector&) const { }
     virtual void ariaControlsElements(AccessibilityChildrenVector&) const { }
     virtual void ariaDescribedbyElements(AccessibilityChildrenVector& describedby) const { };
@@ skipping 171 matching lines @@
     // functions called here may only search up the tree (ancestors), not down.
     void updateCachedAttributeValuesIfNeeded() const;
 };
 
 #define DEFINE_AX_OBJECT_TYPE_CASTS(thisType, predicate) \
     DEFINE_TYPE_CASTS(thisType, AXObject, object, object->predicate, object.predicate)
 
 } // namespace blink
 
 #endif // AXObject_h