Omaha v4 - Add Task Start/Complete to Control Service

Control Service was never terminating due to lacking
the App Server's TaskStarted and TaskCompleted methods.

Change-Id: I16da30af01fbf9a04ec3996dd893548fd3699f2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377147
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801753}

chrome/updater/app/server/mac/service_delegate.mm

@@ -226,8 +226,11 @@
     VLOG(0) << "performControlTasks complete.";
     if (reply)
       reply();
+
+    _appServer->TaskCompleted();
   }));
 
+  _appServer->TaskStarted();
   _callbackRunner->PostTask(
       FROM_HERE,
       base::BindOnce(&updater::ControlService::Run, _service, std::move(cb)));