Introduce PositionAlgorithm<Strategy>::editingPositionOf()

Source/core/editing/commands/DeleteSelectionCommand.cpp

Index: Source/core/editing/commands/DeleteSelectionCommand.cpp
diff --git a/Source/core/editing/commands/DeleteSelectionCommand.cpp b/Source/core/editing/commands/DeleteSelectionCommand.cpp
index 8a6c5686e7a8bdad965445a5fbddc1a1ebf4712a..d2754997ea1395ffed3a21539e0fbc7e7af2b299 100644
--- a/Source/core/editing/commands/DeleteSelectionCommand.cpp
+++ b/Source/core/editing/commands/DeleteSelectionCommand.cpp
@@ -560,7 +560,7 @@ void DeleteSelectionCommand::handleGeneralDelete()
                             offset = n->nodeIndex() + 1;
                     }
                     removeChildrenInRange(m_downstreamEnd.anchorNode(), offset, m_downstreamEnd.computeEditingOffset());
-                    m_downstreamEnd = createLegacyEditingPosition(m_downstreamEnd.anchorNode(), offset);
+                    m_downstreamEnd = Position::editingPositionOf(m_downstreamEnd.anchorNode(), offset);
                 }
             }
         }