Move character{After,Before}() for VisiblePosition to VisibleUnits.cpp

Source/core/editing/VisiblePosition.h

Index: Source/core/editing/VisiblePosition.h
diff --git a/Source/core/editing/VisiblePosition.h b/Source/core/editing/VisiblePosition.h
index 7d6f4cb97c7390a41a4afa690768f77e59ff6344..816051fd25ad35bcf665e79d3cb993228fcb7903 100644
--- a/Source/core/editing/VisiblePosition.h
+++ b/Source/core/editing/VisiblePosition.h
@@ -132,11 +132,6 @@ VisiblePosition rightPositionOf(const VisiblePosition&);
 CORE_EXPORT Position canonicalPositionOf(const Position&);
 CORE_EXPORT PositionInComposedTree canonicalPositionOf(const PositionInComposedTree&);
 
-// TODO(yosin) We should move |characterAfter()| and |characterBefore()| to
-// "VisibleUnits.cpp"
-UChar32 characterAfter(const VisiblePosition&);
-UChar32 characterBefore(const VisiblePosition&);
-
 // TODO(yosin) We should move |honorEditingBoundaryAtOr{Before,After} to
 // "VisibleUnits.cpp" as static function.
 // next() and previous() will increment/decrement by a character cluster.