[B2G] Stub the |DownloadManager| to avoid error

After the refactoring in PR 5678, the B2G viewer now prints the following in the console:
```
ReferenceError: DownloadManager is not defined
```

This will obviously not matter once the B2G viewer is replaced with the new components-based one. But until that happens, I think it makes sense to just stub the class to suppress the error.

web/viewer.js

@@ -65,6 +65,11 @@ var mozL10n = document.mozL10n || document.webL10n;
 //#if GENERIC || CHROME
 //#include download_manager.js
 //#endif
+//#if B2G
+//var DownloadManager = (function DownloadManagerClosure() {
+//  return function DownloadManager() {};
+//})();
+//#endif
 
 //#if FIREFOX || MOZCENTRAL
 //#include firefoxcom.js