Reland "Delay audio service launch on the browser startup."

This is a reland of eab3705f

Original change's description:
> Delay audio service launch on the browser startup.
>
> (see the bug) From the traces the service/process creation gets posted to the
> IO thread during startup.
> It may be delaying startup for some other services needed for the first paint, or it may be
> just eating up CPU when the load is already quite high. We'll experiment with delaying
> the service launch.
>
> Bug: 867461
> Change-Id: I048e33d049e665d8e9489046c5813918d8077524
> Reviewed-on: https://chromium-review.googlesource.com/1153248
> Commit-Queue: Olga Sharonova <olka@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#579460}

Bug: 867461,870171
Change-Id: Ia1a0c31ccc6c643ad10fb250862a9d1fb408c216
Reviewed-on: https://chromium-review.googlesource.com/1162184
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580905}

content/browser/browser_main_loop.cc

@@ -1712,9 +1712,20 @@ void BrowserMainLoop::InitializeAudio() {
   }
 
   if (base::FeatureList::IsEnabled(features::kAudioServiceLaunchOnStartup)) {
-    content::ServiceManagerConnection::GetForProcess()
-        ->GetConnector()
-        ->StartService(audio::mojom::kServiceName);
+    // Schedule the audio service startup on the main thread.
+    BrowserThread::PostAfterStartupTask(
+        FROM_HERE, BrowserThread::GetTaskRunnerForThread(BrowserThread::UI),
+        base::BindOnce([]() {
+          TRACE_EVENT0("audio", "Starting audio service");
+          ServiceManagerConnection* connection =
+              content::ServiceManagerConnection::GetForProcess();
+          if (connection) {
+            // The browser is not shutting down: |connection| would be null
+            // otherwise.
+            connection->GetConnector()->StartService(
+                audio::mojom::kServiceName);
+          }
+        }));
   }
 
   audio_system_ = audio::CreateAudioSystem(