Bug 769467 - Add a PrivateBrowsingUtils module. r=gavin

Add a PrivateBrowsingUtils module that contains a function to find out if an
arbitrary window is a descendant of a private docshell or not.

toolkit/content/Makefile.in

@@ -62,6 +62,7 @@ EXTRA_JS_MODULES = \
   PopupNotifications.jsm \
   Dict.jsm \
   PropertyListUtils.jsm \
+  PrivateBrowsingUtils.jsm \
   $(NULL)
 
 EXTRA_PP_JS_MODULES = \

toolkit/content/PrivateBrowsingUtils.jsm

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+var EXPORTED_SYMBOLS = ["PrivateBrowsingUtils"];
+
+const Ci = Components.interfaces;
+
+var PrivateBrowsingUtils = {
+  isWindowPrivate: function pbu_isWindowPrivate(aWindow) {
+    return aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
+                  .getInterface(Ci.nsIWebNavigation)
+                  .QueryInterface(Ci.nsILoadContext)
+                  .usePrivateBrowsing;
+  }
+};