Bug 1210517 - Create nsVariant directly rather than via do_CreateInstance(). r=froydnj

The goal here is to leave creation stuff mostly for JS, so we can
convert it entirely over to a non-threadsafe cycle-collected version
without breaking any existing C++ users.

I didn't do this for a remaining use in nsGlobalWindow.h to avoid
including nsVariant.h all over the place.

dom/html/TextTrackManager.cpp

@@ -13,6 +13,7 @@
 #include "mozilla/dom/Event.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "nsComponentManagerUtils.h"
+#include "nsVariant.h"
 #include "nsVideoFrame.h"
 #include "nsIFrame.h"
 #include "nsTArrayHelpers.h"
@@ -214,8 +215,7 @@ TextTrackManager::UpdateCueDisplay()
   mTextTracks->UpdateAndGetShowingCues(activeCues);
 
   if (activeCues.Length() > 0) {
-    nsCOMPtr<nsIWritableVariant> jsCues =
-      do_CreateInstance("@mozilla.org/variant;1");
+    nsRefPtr<nsVariant> jsCues = new nsVariant();
 
     jsCues->SetAsArray(nsIDataType::VTYPE_INTERFACE,
                        &NS_GET_IID(nsIDOMEventTarget),