Introduce {left,right}PositionOf() for VisiblePosition

Source/core/editing/VisibleUnits.cpp

Index: Source/core/editing/VisibleUnits.cpp
diff --git a/Source/core/editing/VisibleUnits.cpp b/Source/core/editing/VisibleUnits.cpp
index 3809bb856acaee67334beb2c2357cf47dfb9fc00..e5377ca7909d7b20a9e5d09e05abecd5bf2f1a61 100644
--- a/Source/core/editing/VisibleUnits.cpp
+++ b/Source/core/editing/VisibleUnits.cpp
@@ -375,7 +375,7 @@ static VisiblePosition visualWordPosition(const VisiblePosition& visiblePosition
     Vector<UChar, 1024> string;
 
     while (1) {
-        VisiblePosition adjacentCharacterPosition = direction == MoveRight ? current.right() : current.left();
+        VisiblePosition adjacentCharacterPosition = direction == MoveRight ? rightPositionOf(current) : leftPositionOf(current);
         if (adjacentCharacterPosition.deepEquivalent() == current.deepEquivalent() || adjacentCharacterPosition.isNull())
             return VisiblePosition();