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

Source/core/layout/LayoutView.cpp

Index: Source/core/layout/LayoutView.cpp
diff --git a/Source/core/layout/LayoutView.cpp b/Source/core/layout/LayoutView.cpp
index 02cf3b7ef69509b48842686f84599d70966a069c..6a6419d91d6bbfc57de34fdf7748911a7464bc4a 100644
--- a/Source/core/layout/LayoutView.cpp
+++ b/Source/core/layout/LayoutView.cpp
@@ -803,11 +803,11 @@
     // as the start of the selection, the selection painting code will think that content on the line containing 'foo' is selected
     // and will fill the gap before 'bar'.
     PositionType startPos = Strategy::selectionStart(selection);
-    PositionType candidate = mostBackwardCaretPosition(startPos);
+    PositionType candidate = startPos.downstream();
     if (isVisuallyEquivalentCandidate(candidate))
         startPos = candidate;
     PositionType endPos = Strategy::selectionEnd(selection);
-    candidate = mostForwardCaretPosition(endPos);
+    candidate = endPos.upstream();
     if (isVisuallyEquivalentCandidate(candidate))
         endPos = candidate;