Bug 1517241 - Fix yet another MSVC-only bustage.

This doesn't compile on MSVC for some reason:

```
nsCOMPtr<Foo> f;
RefPtr<Foo> b;

Foo* myFoo = f ? f : b;
```

layout/printing/nsPrintJob.h

@@ -211,7 +211,7 @@ class nsPrintJob final : public nsIObserver,
 
   void PageDone(nsresult aResult);
 
-  nsCOMPtr<mozilla::dom::Document> mDocument;
+  RefPtr<mozilla::dom::Document> mDocument;
   nsCOMPtr<nsIDocumentViewerPrint> mDocViewerPrint;
 
   nsWeakPtr mContainer;