Revert of [Editing][Codehealth] Replace Position::uptream/downstream to mostBackward/ForwardCaretPosition.

Source/core/layout/LayoutObject.cpp

Index: Source/core/layout/LayoutObject.cpp
diff --git a/Source/core/layout/LayoutObject.cpp b/Source/core/layout/LayoutObject.cpp
index b1faf1809cf4e33a37ef7322e8949fa327f8fde3..28c61e1fc62b67b095b35bab84d089161fb73d33 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -2992,10 +2992,10 @@
         if (!node->hasEditableStyle()) {
             // If it can be found, we prefer a visually equivalent position that is editable.
             const Position position = Position(node, offset);
-            Position candidate = mostBackwardCaretPosition(position, CanCrossEditingBoundary);
+            Position candidate = position.downstream(CanCrossEditingBoundary);
             if (candidate.anchorNode()->hasEditableStyle())
                 return PositionWithAffinity(candidate, affinity);
-            candidate = mostForwardCaretPosition(position, CanCrossEditingBoundary);
+            candidate = position.upstream(CanCrossEditingBoundary);
             if (candidate.anchorNode()->hasEditableStyle())
                 return PositionWithAffinity(candidate, affinity);
         }