Refactoring: Remove StyledMarkupAccumulator::appendString and add appendNewLine

Source/core/editing/StyledMarkupAccumulator.h

Index: Source/core/editing/StyledMarkupAccumulator.h
diff --git a/Source/core/editing/StyledMarkupAccumulator.h b/Source/core/editing/StyledMarkupAccumulator.h
index a4a87bece3f4025ad999d354ee5c62722eee9fc5..1248b49be4bcf8f03db1a6f5ebe3ff240f6a9d16 100644
--- a/Source/core/editing/StyledMarkupAccumulator.h
+++ b/Source/core/editing/StyledMarkupAccumulator.h
@@ -50,9 +50,9 @@ public:
 
     bool convertBlocksToInlines() const { return m_convertBlocksToInlines == ConvertBlocksToInlines::Convert; }
 
-    void appendString(const String&);
     void appendStartTag(Node&);
     void appendEndTag(const Element&);
+    void appendNewLine();
 
     // TODO(hajimehoshi): These functions are called from the serializer, but
     // should not.