Move {left,right}PositionOf() 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 28343c14de5e301ff19f66a5dbc6be903cbf6ca5..1ecf859ec616b2ee181819d1659d979068be6c88 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -94,6 +94,9 @@ CORE_EXPORT IntRect absoluteCaretBoundsOf(const VisiblePosition&);
 UChar32 characterAfter(const VisiblePosition&);
 UChar32 characterBefore(const VisiblePosition&);
 
+VisiblePosition leftPositionOf(const VisiblePosition&);
+VisiblePosition rightPositionOf(const VisiblePosition&);
+
 VisiblePosition nextPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);
 VisiblePosition previousPositionOf(const VisiblePosition&, EditingBoundaryCrossingRule = CanCrossEditingBoundary);