Bug 1599379 - Create a DBService instance in GetWorker API when it doesn't exist. r=baku

There are two places using DBService during a page load at startup:
1. nsChannelClassifier::Start, used by Phishing Protection
2. AsyncChannelClassifier::CheckChannel, used by Tracking Protection

Tracking protection checks happen before establishing a network connection, so it happens
prior to phishing protection checkes. When we load a page at
startup, ::CheckChannel API is called, but DBService is not yet created.

This patch fixes this issue by creating a DBService instance when
::GetWorker API is called without a DBService instance.

Differential Revision: https://phabricator.services.mozilla.com/D55248

--HG--
extra : moz-landing-system : lando

toolkit/components/url-classifier/nsUrlClassifierDBService.cpp

@@ -2402,11 +2402,14 @@ bool nsUrlClassifierDBService::ShutdownHasStarted() {
 
 // static
 nsUrlClassifierDBServiceWorker* nsUrlClassifierDBService::GetWorker() {
-  if (!sUrlClassifierDBService) {
+  nsresult rv;
+  RefPtr<nsUrlClassifierDBService> service =
+      nsUrlClassifierDBService::GetInstance(&rv);
+  if (!service) {
     return nullptr;
   }
 
-  return sUrlClassifierDBService->mWorker;
+  return service->mWorker;
 }
 
 NS_IMETHODIMP