Introduce composed tree version of rightPositionOf()

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 854add4194bc23e24a3cdc8dc954d9d6fe00e418..c26174d917c2cbaad9da9c769bf00641760fd2d6 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -101,7 +101,11 @@ UChar32 characterBefore(const VisiblePosition&);
 // composed tree.
 CORE_EXPORT VisiblePosition leftPositionOf(const VisiblePosition&);
 CORE_EXPORT VisiblePositionInComposedTree leftPositionOf(const VisiblePositionInComposedTree&);
-VisiblePosition rightPositionOf(const VisiblePosition&);
+// TODO(yosin) Since return value of |rightPositionOf()| with |VisiblePosition|
+// isn't defined well on composed tree, we should not use it for a position in
+// composed tree.
+CORE_EXPORT VisiblePosition rightPositionOf(const VisiblePosition&);
+CORE_EXPORT VisiblePositionInComposedTree rightPositionOf(const VisiblePositionInComposedTree&);
 
 CORE_EXPORT VisiblePosition nextPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);
 CORE_EXPORT VisiblePositionInComposedTree nextPositionOf(const VisiblePositionInComposedTree&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);