Fix GC race in test by doing QueryInterface when needed, in case a the wrapper has been GCed since the last QueryInterface. (Bug 600785) r=ted.mielczarek a2.0=tests

toolkit/crashreporter/test/browser/aboutcrashes_utils.js

@@ -57,7 +57,8 @@ function make_fake_appdir() {
 function cleanup_fake_appdir() {
   let dirSvc = Cc["@mozilla.org/file/directory_service;1"]
                .getService(Ci.nsIProperties);
-  dirSvc.unregisterProvider(_provider);
+  dirSvc.QueryInterface(Ci.nsIDirectoryService)
+        .unregisterProvider(_provider);
   // undefine our value so future calls get the real value
   try {
     dirSvc.undefine("UAppData");