Use stricter typing for NodeLists throughout the code base

Source/core/html/HTMLFormElement.h

Index: Source/core/html/HTMLFormElement.h
diff --git a/Source/core/html/HTMLFormElement.h b/Source/core/html/HTMLFormElement.h
index 6c63e6f1141392aceab16768d64de6067ddb41b4..818bf0a983cbd066e0ef2e814eb67276aeabc4df 100644
--- a/Source/core/html/HTMLFormElement.h
+++ b/Source/core/html/HTMLFormElement.h
@@ -26,6 +26,7 @@
 
 #include "core/html/HTMLElement.h"
 #include "core/html/HTMLFormControlElement.h"
+#include "core/html/HTMLFormControlsCollection.h"
 #include "core/html/forms/RadioButtonGroupScope.h"
 #include "core/loader/FormSubmission.h"
 #include "wtf/OwnPtr.h"
@@ -51,7 +52,7 @@ public:
     virtual ~HTMLFormElement();
     virtual void trace(Visitor*) OVERRIDE;
 
-    PassRefPtrWillBeRawPtr<HTMLCollection> elements();
+    PassRefPtrWillBeRawPtr<HTMLFormControlsCollection> elements();
     void getNamedElements(const AtomicString&, WillBeHeapVector<RefPtrWillBeMember<Element> >&);
 
     unsigned length() const;