Add comment about CaretBase::updateCaretRect.

Source/core/editing/Caret.h

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 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 30 matching lines @@
 
 class CaretBase {
     WTF_MAKE_NONCOPYABLE(CaretBase);
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED;
 protected:
     enum CaretVisibility { Visible, Hidden };
     explicit CaretBase(CaretVisibility = Hidden);
 
     void invalidateCaretRect(Node*, bool caretRectChanged = false);
     void clearCaretRect();
+    // Creating VisiblePosition causes synchronous layout so we should use the
+    // PositionWithAffinity version if possible.
+    // A position in HTMLTextFromControlElement is a typical example.
     bool updateCaretRect(Document*, const PositionWithAffinity& caretPosition);
-    // Simply calls above updateCaretRect using deepEquivalent() and affinity().
     bool updateCaretRect(Document*, const VisiblePosition& caretPosition);
     IntRect absoluteBoundsForLocalRect(Node*, const LayoutRect&) const;
     bool shouldRepaintCaret(const RenderView*, bool isContentEditable) const;
     void paintCaret(Node*, GraphicsContext*, const LayoutPoint&, const LayoutRect& clipRect) const;
 
     const LayoutRect& localCaretRectWithoutUpdate() const { return m_caretLocalRect; }
 
     bool shouldUpdateCaretRect() const { return m_caretRectNeedsUpdate; }
     void setCaretRectNeedsUpdate() { m_caretRectNeedsUpdate = true; }
 
@@ skipping 35 matching lines @@
 private:
     DragCaretController();
 
     VisiblePosition m_position;
 };
 
 } // namespace blink
 
 
 #endif // Caret_h