Move member function rootEditableElement() out from VisiblePosition class

Source/core/editing/DragCaretController.cpp

 /*
  * Copyright (C) 2004, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 89 matching lines @@
 DEFINE_TRACE(DragCaretController)
 {
     visitor->trace(m_position);
 }
 
 LayoutBlock* DragCaretController::caretLayoutObject() const
 {
     return CaretBase::caretLayoutObject(m_position.deepEquivalent().anchorNode());
 }
 
+bool DragCaretController::isContentEditable() const
+{
+    return rootEditableElementOf(m_position);
+}
+
 void DragCaretController::paintDragCaret(LocalFrame* frame, GraphicsContext* p, const LayoutPoint& paintOffset, const LayoutRect& clipRect) const
 {
     if (m_position.deepEquivalent().anchorNode()->document().frame() == frame)
         paintCaret(m_position.deepEquivalent().anchorNode(), p, paintOffset, clipRect);
 }
 
 } // namespace blink