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/HTMLInputElement.cpp

@@ -63,6 +63,7 @@
 #include "nsReadableUtils.h"
 #include "nsUnicharUtils.h"
 #include "nsLayoutUtils.h"
+#include "nsVariant.h"
 
 #include "nsIDOMMutationEvent.h"
 #include "mozilla/ContentEvents.h"
@@ -807,9 +808,7 @@ UploadLastDir::StoreLastUsedDirectory(nsIDocument* aDoc, nsIFile* aDir)
   aDir->GetPath(unicodePath);
   if (unicodePath.IsEmpty()) // nothing to do
     return NS_OK;
-  nsCOMPtr<nsIWritableVariant> prefValue = do_CreateInstance(NS_VARIANT_CONTRACTID);
-  if (!prefValue)
-    return NS_ERROR_OUT_OF_MEMORY;
+  nsRefPtr<nsVariant> prefValue = new nsVariant();
   prefValue->SetAsAString(unicodePath);
 
   // Use the document's current load context to ensure that the content pref