Get rid of redundant member function PositionAlgorithm<Strategy>::{upstream,downstream}

Source/core/editing/commands/InsertTextCommand.cpp

 /*
  * Copyright (C) 2005 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 138 matching lines @@
             return;
     }
 
     Position startPosition(endingSelection().start());
 
     Position placeholder;
     // We want to remove preserved newlines and brs that will collapse (and thus become unnecessary) when content
     // is inserted just before them.
     // FIXME: We shouldn't really have to do this, but removing placeholders is a workaround for 9661.
     // If the caret is just before a placeholder, downstream will normalize the caret to it.
-    Position downstream(startPosition.downstream());
+    Position downstream(mostForwardCaretPosition(startPosition));
     if (lineBreakExistsAtPosition(downstream)) {
         // FIXME: This doesn't handle placeholders at the end of anonymous blocks.
         VisiblePosition caret(startPosition);
         if (isEndOfBlock(caret) && isStartOfParagraph(caret))
             placeholder = downstream;
         // Don't remove the placeholder yet, otherwise the block we're inserting into would collapse before
         // we get a chance to insert into it.  We check for a placeholder now, though, because doing so requires
         // the creation of a VisiblePosition, and if we did that post-insertion it would force a layout.
     }
 
     // Insert the character at the leftmost candidate.
-    startPosition = startPosition.upstream();
+    startPosition = mostBackwardCaretPosition(startPosition);
 
     // It is possible for the node that contains startPosition to contain only unrendered whitespace,
     // and so deleteInsignificantText could remove it.  Save the position before the node in case that happens.
     ASSERT(startPosition.computeContainerNode());
     Position positionBeforeStartNode(positionInParentBeforeNode(*startPosition.computeContainerNode()));
-    deleteInsignificantText(startPosition, startPosition.downstream());
+    deleteInsignificantText(startPosition, mostForwardCaretPosition(startPosition));
     if (!startPosition.inDocument())
         startPosition = positionBeforeStartNode;
     if (!isVisuallyEquivalentCandidate(startPosition))
-        startPosition = startPosition.downstream();
+        startPosition = mostForwardCaretPosition(startPosition);
 
     startPosition = positionAvoidingSpecialElementBoundary(startPosition);
 
     Position endPosition;
 
     if (m_text == "\t") {
         endPosition = insertTab(startPosition);
         // TODO(yosin) We should use |PositionMoveType::Character| for
         // |previousPositionOf()|.
         startPosition = previousPositionOf(endPosition, PositionMoveType::Character);
@@ skipping 74 matching lines @@
                 splitTextNode(textNode, offset);
             insertNodeBefore(spanElement, textNode.release());
         }
     }
 
     // return the position following the new tab
     return lastPositionInNode(spanElement.get());
 }
 
 }