Make TextIterator to stop when it falls to handle shadow tree

Source/core/editing/iterators/TextIterator.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2005 Alexey Proskuryakov.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 350 matching lines @@
                     if (m_textState.positionNode()) {
                         m_iterationProgress = HandledChildren;
                         return;
                     }
                     next = Strategy::nextSibling(*m_node);
                 }
 
                 if (!next && !parentNode && m_shadowDepth > 0) {
                     // 4. Reached the top of a shadow root. If it's created by author, then try to visit the next
                     // sibling shadow root, if any.
+                    if (!m_node->isShadowRoot()) {
+                        ASSERT_NOT_REACHED();
+                        m_shouldStop = true;
+                        return;
+                    }
                     ShadowRoot* shadowRoot = toShadowRoot(m_node);
                     if (shadowRoot->type() == ShadowRootType::OpenByDefault || shadowRoot->type() == ShadowRootType::Open) {
                         ShadowRoot* nextShadowRoot = shadowRoot->olderShadowRoot();
                         if (nextShadowRoot && nextShadowRoot->type() == ShadowRootType::OpenByDefault) {
                             m_fullyClippedStack.pop();
                             m_node = nextShadowRoot;
                             m_iterationProgress = HandledNone;
                             // m_shadowDepth is unchanged since we exit from a shadow root and enter another.
                             m_fullyClippedStack.pushFullyClippedState(m_node);
                         } else {
@@ skipping 732 matching lines @@
 
 String plainText(const EphemeralRangeInComposedTree& range, TextIteratorBehaviorFlags behavior)
 {
     return createPlainText<EditingInComposedTreeStrategy>(range, behavior);
 }
 
 template class CORE_TEMPLATE_EXPORT TextIteratorAlgorithm<EditingStrategy>;
 template class CORE_TEMPLATE_EXPORT TextIteratorAlgorithm<EditingInComposedTreeStrategy>;
 
 } // namespace blink