Introduce {next,previous}PositionOf() as replacement of {next,previous}() member functions in Positi

Source/core/editing/commands/InsertTextCommand.cpp

Index: Source/core/editing/commands/InsertTextCommand.cpp
diff --git a/Source/core/editing/commands/InsertTextCommand.cpp b/Source/core/editing/commands/InsertTextCommand.cpp
index 48476722a91e18aaf6f4aed7e80aebf8aac86803..e5d289914f1a60824d40f6c4d298d0665a4bff3b 100644
--- a/Source/core/editing/commands/InsertTextCommand.cpp
+++ b/Source/core/editing/commands/InsertTextCommand.cpp
@@ -186,7 +186,9 @@ void InsertTextCommand::doApply()
 
     if (m_text == "\t") {
         endPosition = insertTab(startPosition);
-        startPosition = endPosition.previous();
+        // TODO(yosin) We should use |PositionMoveType::Character| for
+        // |previousPositionOf()|.
+        startPosition = previousPositionOf(endPosition, PositionMoveType::Character);
         if (placeholder.isNotNull())
             removePlaceholderAt(placeholder);
     } else {