Move character{After,Before}() 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 d463928adc1152750294bae59a2c7aa59b083f6f..d0ba219bbfff96479f5659cbdd3f923b962d00d8 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&);
 
+UChar32 characterAfter(const VisiblePosition&);
+UChar32 characterBefore(const VisiblePosition&);
+
 VisiblePosition nextPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);
 VisiblePosition previousPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);