Use new Traversal<> API more in the code base

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

Index: Source/core/html/track/vtt/VTTRegion.cpp
diff --git a/Source/core/html/track/vtt/VTTRegion.cpp b/Source/core/html/track/vtt/VTTRegion.cpp
index d2bc0555291250fa36c8f3b8a7fb8f540a688bb2..79f8164fca4fc3f3bd06d09e98757b63ca704176 100644
--- a/Source/core/html/track/vtt/VTTRegion.cpp
+++ b/Source/core/html/track/vtt/VTTRegion.cpp
@@ -374,8 +374,9 @@ void VTTRegion::displayLastVTTCueBox()
 
     // 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;