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..1639ab6b130b44ca655e887abdd59a4fe6492ec0 100644
--- a/Source/core/html/track/vtt/VTTRegion.cpp
+++ b/Source/core/html/track/vtt/VTTRegion.cpp
@@ -373,9 +373,9 @@ void VTTRegion::displayLastVTTCueBox()
     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();
+    for (HTMLDivElement* child = Traversal<HTMLDivElement>::firstChild(*m_cueContainer); child && !m_scrollTimer.isActive(); child = Traversal<HTMLDivElement>::nextSibling(*child)) {

[adamk, 2014/07/28 21:06:38]

It's not clear to me why this code was calling toHTMLDivElement() before;
getBoundingClientRect() is on Element. It seems like it might be better off
sticking with ElementTraversal.

[Inactive, 2014/07/28 21:13:33]

Yes, this looked unsafe. OK, I'll keep using ElementTraversal here and merely
get rid of the toHMTMLDivElement() cast then.

[Inactive, 2014/07/28 21:41:06]

Done.

+        float childTop = child->getBoundingClientRect()->top();
+        float childBottom = child->getBoundingClientRect()->bottom();

[adamk, 2014/07/28 21:06:38]

Sad that this code calls getBoundingClientRect() twice. While you're here, would
you mind storing the return value and accessing top and bottom from it?

[Inactive, 2014/07/28 21:41:06]

Done.

 
         if (regionBottom >= childBottom)
             continue;