Move {next,previous}PositionOf() 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 0f634e5d7bb457eedee1db01c80275ec20b65da7..d463928adc1152750294bae59a2c7aa59b083f6f 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&);
 
+VisiblePosition nextPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);
+VisiblePosition previousPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);
+
 // words
 CORE_EXPORT VisiblePosition startOfWord(const VisiblePosition&, EWordSide = RightWordIfOnBoundary);
 CORE_EXPORT VisiblePosition endOfWord(const VisiblePosition&, EWordSide = RightWordIfOnBoundary);