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..b4fb4793566c79379ac0fc81c9e664f76dc892a0 100644
--- a/Source/core/html/HTMLFormElement.h
+++ b/Source/core/html/HTMLFormElement.h
@@ -42,6 +42,7 @@ class FormAssociatedElement;
 class FormData;
 class GenericEventQueue;
 class HTMLFormControlElement;
+class HTMLFormControlsCollection;
 class HTMLImageElement;
 class HTMLInputElement;
 
@@ -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;