Use stricter typing for NodeLists throughout the code base

Source/core/html/LabelableElement.h

Index: Source/core/html/LabelableElement.h
diff --git a/Source/core/html/LabelableElement.h b/Source/core/html/LabelableElement.h
index 59b3ccf4efe407720ed969972b081153423e4594..cc0c3362b627390b66589b0c78f6089482eeaa69 100644
--- a/Source/core/html/LabelableElement.h
+++ b/Source/core/html/LabelableElement.h
@@ -32,6 +32,7 @@
 #define LabelableElement_h
 
 #include "core/html/HTMLElement.h"
+#include "core/html/LabelsNodeList.h"
 
 namespace WebCore {
 
@@ -40,7 +41,7 @@ namespace WebCore {
 class LabelableElement : public HTMLElement {
 public:
     virtual ~LabelableElement();
-    PassRefPtrWillBeRawPtr<NodeList> labels();
+    PassRefPtrWillBeRawPtr<LabelsNodeList> labels();
     virtual bool supportLabels() const { return false; }
 
     virtual void trace(Visitor*) OVERRIDE;