diff -r 000000000000 -r 6474c204b198 js/xpconnect/src/xpcObjectHelper.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/xpconnect/src/xpcObjectHelper.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,134 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* vim: set ts=8 sts=4 et sw=4 tw=99: */
+/* 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/. */
+
+#ifndef xpcObjectHelper_h
+#define xpcObjectHelper_h
+
+// Including 'windows.h' will #define GetClassInfo to something else.
+#ifdef XP_WIN
+#ifdef GetClassInfo
+#undef GetClassInfo
+#endif
+#endif
+
+#include "mozilla/Attributes.h"
+#include <stdint.h>
+#include "nsAutoPtr.h"
+#include "nsCOMPtr.h"
+#include "nsIClassInfo.h"
+#include "nsISupports.h"
+#include "nsIXPCScriptable.h"
+#include "nsWrapperCache.h"
+
+class xpcObjectHelper
+{
+public:
+    xpcObjectHelper(nsISupports *aObject, nsWrapperCache *aCache = nullptr)
+      : mCanonical(nullptr)
+      , mObject(aObject)
+      , mCache(aCache)
+    {
+        if (!mCache) {
+            if (aObject)
+                CallQueryInterface(aObject, &mCache);
+            else
+                mCache = nullptr;
+        }
+    }
+
+    nsISupports *Object()
+    {
+        return mObject;
+    }
+
+    nsISupports *GetCanonical()
+    {
+        if (!mCanonical) {
+            mCanonicalStrong = do_QueryInterface(mObject);
+            mCanonical = mCanonicalStrong;
+        }
+        return mCanonical;
+    }
+
+    already_AddRefed<nsISupports> forgetCanonical()
+    {
+        MOZ_ASSERT(mCanonical, "Huh, no canonical to forget?");
+
+        if (!mCanonicalStrong)
+            mCanonicalStrong = mCanonical;
+        mCanonical = nullptr;
+        return mCanonicalStrong.forget();
+    }
+
+    nsIClassInfo *GetClassInfo()
+    {
+        if (mXPCClassInfo)
+          return mXPCClassInfo;
+        if (!mClassInfo)
+            mClassInfo = do_QueryInterface(mObject);
+        return mClassInfo;
+    }
+    nsXPCClassInfo *GetXPCClassInfo()
+    {
+        if (!mXPCClassInfo) {
+            CallQueryInterface(mObject, getter_AddRefs(mXPCClassInfo));
+        }
+        return mXPCClassInfo;
+    }
+
+    already_AddRefed<nsXPCClassInfo> forgetXPCClassInfo()
+    {
+        GetXPCClassInfo();
+
+        return mXPCClassInfo.forget();
+    }
+
+    // We assert that we can reach an nsIXPCScriptable somehow.
+    uint32_t GetScriptableFlags()
+    {
+        // Try getting an nsXPCClassInfo - this handles DOM scriptable helpers.
+        nsCOMPtr<nsIXPCScriptable> sinfo = GetXPCClassInfo();
+
+        // If that didn't work, try just QI-ing. This handles BackstagePass.
+        if (!sinfo)
+            sinfo = do_QueryInterface(GetCanonical());
+
+        // We should have something by now.
+        MOZ_ASSERT(sinfo);
+
+        // Grab the flags.
+        return sinfo->GetScriptableFlags();
+    }
+
+    nsWrapperCache *GetWrapperCache()
+    {
+        return mCache;
+    }
+
+protected:
+    xpcObjectHelper(nsISupports *aObject, nsISupports *aCanonical,
+                    nsWrapperCache *aCache)
+      : mCanonical(aCanonical)
+      , mObject(aObject)
+      , mCache(aCache)
+    {
+        if (!mCache && aObject)
+            CallQueryInterface(aObject, &mCache);
+    }
+
+    nsCOMPtr<nsISupports>    mCanonicalStrong;
+    nsISupports*             mCanonical;
+
+private:
+    xpcObjectHelper(xpcObjectHelper& aOther) MOZ_DELETE;
+
+    nsISupports*             mObject;
+    nsWrapperCache*          mCache;
+    nsCOMPtr<nsIClassInfo>   mClassInfo;
+    nsRefPtr<nsXPCClassInfo> mXPCClassInfo;
+};
+
+#endif