No bug - Fix non-unified build in nsImageBoxFrame.cpp

Differential Revision: https://phabricator.services.mozilla.com/D62147

--HG--
extra : moz-landing-system : lando

layout/xul/nsImageBoxFrame.cpp

@@ -65,6 +65,10 @@ using namespace mozilla::gfx;
 using namespace mozilla::image;
 using namespace mozilla::layers;
 
+using mozilla::dom::Element;
+using mozilla::dom::Document;
+using mozilla::dom::ReferrerInfo;
+
 class nsImageBoxFrameEvent : public Runnable {
  public:
   nsImageBoxFrameEvent(nsIContent* content, EventMessage message)