Bug 1479860: Add a missing override keyword that got lost. r=me

on a CLOSED TREE of course, sigh.

MozReview-Commit-ID: 5nqGog4zg3Z

dom/html/HTMLLegendElement.h

@@ -33,7 +33,7 @@ public:
   // nsIContent
   virtual nsresult BindToTree(nsIDocument* aDocument,
                               nsIContent* aParent,
-                              nsIContent* aBindingParent);
+                              nsIContent* aBindingParent) override;
   virtual void UnbindFromTree(bool aDeep = true,
                               bool aNullParent = true) override;
   virtual bool ParseAttribute(int32_t aNamespaceID,