Move {left,right}PositionOf() 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 db1896f1ffe1733bdb0404c8524299fbe027ac85..29493c0515b42e5d861a9fe24dfdf56f14b9e9d9 100644
--- a/Source/core/editing/VisiblePosition.h
+++ b/Source/core/editing/VisiblePosition.h
@@ -112,11 +112,6 @@ private:
     TextAffinity m_affinity;
 };
 
-// TODO(yosin) We should move |leftPositionOf()| and |rightPositionOf()| to
-// "VisibleUnits.cpp".
-VisiblePosition leftPositionOf(const VisiblePosition&);
-VisiblePosition rightPositionOf(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.