Move a member function parentEditingBoundary() out from PositionAlgorithm<Strategy> template class

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 f794a3fe20e6200be42ea711543987b11b365dd8..e4835dd67e130a26cf25b698115484ec5ef85db5 100644
--- a/Source/core/editing/Position.h
+++ b/Source/core/editing/Position.h
@@ -182,8 +182,6 @@ public:
     bool atFirstEditingPositionForNode() const;
     bool atLastEditingPositionForNode() const;
 
-    Node* parentEditingBoundary() const;
-
     bool atStartOfTree() const;
     bool atEndOfTree() const;
 
@@ -252,6 +250,11 @@ using PositionInComposedTree = PositionAlgorithm<EditingInComposedTreeStrategy>;
 // since it is used only in "editing/commands/"
 CORE_EXPORT bool isRenderedCharacter(const Position&);
 
+// 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)
 {