--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -4085,16 +4085,29 @@ nsGlobalWindow::GetOwnPropertyNames(JSCo
                                     ErrorResult& aRv)
 {
   nsScriptNameSpaceManager* nameSpaceManager = GetNameSpaceManager();
   if (nameSpaceManager) {
     nameSpaceManager->EnumerateGlobalNames(EnumerateGlobalName, &aNames);
   }
 }
 
+/* static */ bool
+nsGlobalWindow::IsChromeWindow(JSContext* aCx, JSObject* aObj)
+{
+  // For now, have to deal with XPConnect objects here.
+  nsGlobalWindow* win;
+  nsresult rv = UNWRAP_OBJECT(Window, aObj, win);
+  if (NS_FAILED(rv)) {
+    nsCOMPtr<nsPIDOMWindow> piWin = do_QueryWrapper(aCx, aObj);
+    win = static_cast<nsGlobalWindow*>(piWin.get());
+  }
+  return win->IsChromeWindow();
+}
+
 nsIDOMOfflineResourceList*
 nsGlobalWindow::GetApplicationCache(ErrorResult& aError)
 {
   FORWARD_TO_INNER_OR_THROW(GetApplicationCache, (aError), aError, nullptr);
 
   if (!mApplicationCache) {
     nsCOMPtr<nsIWebNavigation> webNav(do_QueryInterface(GetDocShell()));
     if (!webNav) {

--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -470,16 +470,18 @@ public:
   // nsIInterfaceRequestor
   NS_DECL_NSIINTERFACEREQUESTOR
 
   // WebIDL interface.
   already_AddRefed<nsIDOMWindow> IndexedGetter(uint32_t aIndex, bool& aFound);
 
   void GetSupportedNames(nsTArray<nsString>& aNames);
 
+  static bool IsChromeWindow(JSContext* /* unused */, JSObject* aObj);
+
   bool DoNewResolve(JSContext* aCx, JS::Handle<JSObject*> aObj,
                     JS::Handle<jsid> aId,
                     JS::MutableHandle<JSPropertyDescriptor> aDesc);
 
   void GetOwnPropertyNames(JSContext* aCx, nsTArray<nsString>& aNames,
                            mozilla::ErrorResult& aRv);
 
   // Object Management