Introduce previousPositionOf() for VisiblePosition

Source/core/editing/serializers/StyledMarkupSerializer.cpp

Index: Source/core/editing/serializers/StyledMarkupSerializer.cpp
diff --git a/Source/core/editing/serializers/StyledMarkupSerializer.cpp b/Source/core/editing/serializers/StyledMarkupSerializer.cpp
index 86205e83a8334c968a8a7bb404feaa556c3519dd..23a3a5591590e51159fa52ea6152d26f8aa9882c 100644
--- a/Source/core/editing/serializers/StyledMarkupSerializer.cpp
+++ b/Source/core/editing/serializers/StyledMarkupSerializer.cpp
@@ -145,9 +145,9 @@ static bool needInterchangeNewlineAfter(const VisiblePosition& v)
 
 static bool needInterchangeNewlineAt(const VisiblePosition& v)
 {
-    // FIXME: |v.previous()| works on a DOM tree. We need to fix this to work on
-    // a composed tree.
-    return needInterchangeNewlineAfter(v.previous());
+    // TODO(yosin) |previousPositionOf(v)| works on a DOM tree. We need to fix
+    // this to work on a composed tree.
+    return needInterchangeNewlineAfter(previousPositionOf(v));
 }
 
 template<typename Strategy>
@@ -179,7 +179,7 @@ String StyledMarkupSerializer<Strategy>::createMarkup()
     VisiblePosition visibleEnd(m_end);
     if (shouldAnnotate() && needInterchangeNewlineAfter(visibleStart)) {
         markupAccumulator.appendInterchangeNewline();
-        if (visibleStart.deepEquivalent() == visibleEnd.previous().deepEquivalent())
+        if (visibleStart.deepEquivalent() == previousPositionOf(visibleEnd).deepEquivalent())
             return markupAccumulator.takeResults();
 
         firstNode = visibleStart.next().deepEquivalent().anchorNode();