Introduce use counters for shadow DOM handling in plainText()

Source/core/editing/iterators/TextIterator.h

Index: Source/core/editing/iterators/TextIterator.h
diff --git a/Source/core/editing/iterators/TextIterator.h b/Source/core/editing/iterators/TextIterator.h
index 39a8d236e32344f30ba98157cc9b4027d011eda8..589181b675cfc00251ade54ab899b933a45c8682 100644
--- a/Source/core/editing/iterators/TextIterator.h
+++ b/Source/core/editing/iterators/TextIterator.h
@@ -178,6 +178,9 @@ private:
     bool m_handledFirstLetter;
     // Used when stopsOnFormControls() is true to determine if the iterator should keep advancing.
     bool m_shouldStop;
+    // Used for use counter |InnerTextWithShadowTree| and
+    // |SelectionToStringWithShadowTree|, we should not use other purpose.
+    bool m_handleShadowRoot;
 
     // Contains state of emitted text.
     TextIteratorTextState m_textState;