Introduce composed tree version of canonicalPosition() in VisiblePosition

Source/core/editing/VisiblePosition.h

 /*
  * Copyright (C) 2004, 2008 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 123 matching lines @@
 }
 
 PassRefPtrWillBeRawPtr<Range> makeRange(const VisiblePosition&, const VisiblePosition&);
 bool setStart(Range*, const VisiblePosition&);
 bool setEnd(Range*, const VisiblePosition&);
 VisiblePosition startVisiblePosition(const Range*, EAffinity);
 
 Element* enclosingBlockFlowElement(const VisiblePosition&);
 
 Position canonicalPositionOf(const Position&);
+PositionInComposedTree canonicalPositionOf(const PositionInComposedTree&);
 PositionWithAffinity honorEditingBoundaryAtOrBeforeOf(const PositionWithAffinity&, const Position& anchor);
 
 bool isFirstVisiblePositionInNode(const VisiblePosition&, const ContainerNode*);
 bool isLastVisiblePositionInNode(const VisiblePosition&, const ContainerNode*);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const blink::VisiblePosition*);
 void showTree(const blink::VisiblePosition&);
 #endif
 
 #endif // VisiblePosition_h