Bug 1315140 - Add telemetry for the Safe Browsing shutdown code. r=francois

MozReview-Commit-ID: F9Hv37Mn73P

toolkit/components/url-classifier/nsUrlClassifierDBService.cpp

@@ -1737,6 +1737,8 @@ nsUrlClassifierDBService::Shutdown()
   if (!gDbBackgroundThread)
     return NS_OK;
 
+  Telemetry::AutoTimer<Telemetry::URLCLASSIFIER_SHUTDOWN_TIME> timer;
+
   mCompleters.Clear();
 
   nsCOMPtr<nsIPrefBranch> prefs = do_GetService(NS_PREFSERVICE_CONTRACTID);