Move parentEditingBoundary() to VisibileUnits.cpp from Position.cpp.

Source/core/editing/Position.h

Index: Source/core/editing/Position.h
diff --git a/Source/core/editing/Position.h b/Source/core/editing/Position.h
index 6dd1d1093669eab44dc20220cf7e2c0a97f67c6f..cb8a0bc5dcc0fad9abbbd07eee98e70c73f4d6d6 100644
--- a/Source/core/editing/Position.h
+++ b/Source/core/editing/Position.h
@@ -238,11 +238,6 @@ extern template class CORE_EXTERN_TEMPLATE_EXPORT PositionAlgorithm<EditingInCom
 using Position = PositionAlgorithm<EditingStrategy>;
 using PositionInComposedTree = PositionAlgorithm<EditingInComposedTreeStrategy>;
 
-// TODO(yosin) We should make |parentEditingBoundary()| as static function
-// in "VisibleUnits.cpp", since it is used only there.
-CORE_EXPORT Node* parentEditingBoundary(const Position&);
-CORE_EXPORT Node* parentEditingBoundary(const PositionInComposedTree&);
-
 template <typename Strategy>
 bool operator==(const PositionAlgorithm<Strategy>& a, const PositionAlgorithm<Strategy>& b)
 {