Use stricter typing for NodeLists throughout the code base

Source/core/html/HTMLFieldSetElement.cpp

Index: Source/core/html/HTMLFieldSetElement.cpp
diff --git a/Source/core/html/HTMLFieldSetElement.cpp b/Source/core/html/HTMLFieldSetElement.cpp
index 46a56b3bb67d9f8bf519a64fddd358ec238281fd..54f6d7bbb93e27caae875acd812cb06060b13c13 100644
--- a/Source/core/html/HTMLFieldSetElement.cpp
+++ b/Source/core/html/HTMLFieldSetElement.cpp
@@ -28,6 +28,7 @@
 #include "core/HTMLNames.h"
 #include "core/dom/ElementTraversal.h"
 #include "core/html/HTMLCollection.h"
+#include "core/html/HTMLFormControlsCollection.h"
 #include "core/html/HTMLLegendElement.h"
 #include "core/html/HTMLObjectElement.h"
 #include "core/rendering/RenderFieldset.h"
@@ -100,9 +101,9 @@ HTMLLegendElement* HTMLFieldSetElement::legend() const
     return Traversal<HTMLLegendElement>::firstChild(*this);
 }
 
-PassRefPtrWillBeRawPtr<HTMLCollection> HTMLFieldSetElement::elements()
+PassRefPtrWillBeRawPtr<HTMLFormControlsCollection> HTMLFieldSetElement::elements()
 {
-    return ensureCachedHTMLCollection(FormControls);
+    return toHTMLFormControlsCollection(ensureCachedHTMLCollection(FormControls).get());
 }
 
 void HTMLFieldSetElement::refreshElementsIfNeeded() const