Use new Traversal<> API more in the code base

Source/core/html/track/vtt/VTTRegion.cpp

 /*
  * Copyright (C) 2013 Google Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 356 matching lines @@
         return;
 
     // If it's a scrolling region, add the scrolling class.
     if (isScrollingRegion())
         m_cueContainer->classList().add(textTrackCueContainerScrollingClass(), ASSERT_NO_EXCEPTION);
 
     float regionBottom = m_regionDisplayTree->getBoundingClientRect()->bottom();
 
     // Find first cue that is not entirely displayed and scroll it upwards.
     for (Element* child = ElementTraversal::firstChild(*m_cueContainer); child && !m_scrollTimer.isActive(); child = ElementTraversal::nextSibling(*child)) {
-        float childTop = toHTMLDivElement(child)->getBoundingClientRect()->top();
-        float childBottom = toHTMLDivElement(child)->getBoundingClientRect()->bottom();
+        RefPtrWillBeRawPtr<ClientRect> clientRect = child->getBoundingClientRect();
+        float childTop = clientRect->top();
+        float childBottom = clientRect->bottom();
 
         if (regionBottom >= childBottom)
             continue;
 
         float height = childBottom - childTop;
 
         m_currentTop -= std::min(height, childBottom - regionBottom);
         m_cueContainer->setInlineStyleProperty(CSSPropertyTop, m_currentTop, CSSPrimitiveValue::CSS_PX);
 
         startTimer();
@@ skipping 81 matching lines @@
 }
 
 void VTTRegion::trace(Visitor* visitor)
 {
     visitor->trace(m_cueContainer);
     visitor->trace(m_regionDisplayTree);
     visitor->trace(m_track);
 }
 
 } // namespace blink