Update "Hooking Up the Service Implementation"

Looks like service.cc now registers new out-of-process services
differently. Changing the code snippet accordingly.

Bug: None
Change-Id: Iac9b87d5f62006e24a48d3f8fa3feff3ab1fcaad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536433
Auto-Submit: Miriam Polzer <mpolzer@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#827302}

docs/mojo_and_services.md

@@ -368,14 +368,13 @@ auto RunMathService(mojo::PendingReceiver<math::mojom::MathService> receiver) {
   return std::make_unique<math::MathService>(std::move(receiver));
 }
 
-mojo::ServiceFactory* GetMainThreadServiceFactory() {
-  // Existing factories...
-  static base::NoDestructor<mojo::ServiceFactory> factory {
-    RunFilePatcher,
-    RunUnzipper,
+void RegisterMainThreadServices(mojo::ServiceFactory& services) {
+  // Existing services...
+  services.Add(RunFilePatcher);
+  services.Add(RunUnzipper);
 
   // We add our own factory to this list
-    RunMathService,
+  services.Add(RunMathService);
   //...
 ```