Move canonicalPosition() for VisiblePosition to VisibleUnits.cpp

Source/core/editing/VisibleUnits.h

Index: Source/core/editing/VisibleUnits.h
diff --git a/Source/core/editing/VisibleUnits.h b/Source/core/editing/VisibleUnits.h
index d0ba219bbfff96479f5659cbdd3f923b962d00d8..064085042967b3d69e8b3bfc57df1b385594412f 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -85,6 +85,9 @@ CORE_EXPORT PositionInComposedTree mostForwardCaretPosition(const PositionInComp
 CORE_EXPORT bool isVisuallyEquivalentCandidate(const Position&);
 CORE_EXPORT bool isVisuallyEquivalentCandidate(const PositionInComposedTree&);
 
+CORE_EXPORT Position canonicalPositionOf(const Position&);
+CORE_EXPORT PositionInComposedTree canonicalPositionOf(const PositionInComposedTree&);
+
 UChar32 characterAfter(const VisiblePosition&);
 UChar32 characterBefore(const VisiblePosition&);