Introduce use counters for shadow DOM handling in plainText()

Source/core/editing/DOMSelection.cpp

Index: Source/core/editing/DOMSelection.cpp
diff --git a/Source/core/editing/DOMSelection.cpp b/Source/core/editing/DOMSelection.cpp
index 9b86b6fa4dccbec1bd0dfd3b5914847dedda6451..63d3f9261afdae84dfb6a72c7dc2b6ad07f74488 100644
--- a/Source/core/editing/DOMSelection.cpp
+++ b/Source/core/editing/DOMSelection.cpp
@@ -505,7 +505,7 @@ String DOMSelection::toString()
 
     Position start, end;
     if (m_frame->selection().selection().toNormalizedPositions(start, end))
-        return plainText(start, end);
+        return plainText(start, end, TextIteratorForSelectionToString);
     return emptyString();
 }