Feature flag to launch the audio service on the browser startup.

Audio process experiment is observing performance regressions caused by
on-the-flight audio process launch. Introducing a feature flag to
pre-launch it on the browser startup.
This will be rolled out as another AudioService experiment group, to
evaluate memory and performance impact.

TEST=manual

Bug: 851611,853781
Change-Id: I30c1798dae07c5ce8a71243ab482ceb89d008e43
Reviewed-on: https://chromium-review.googlesource.com/1104681
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568389}

content/browser/browser_main_loop.cc

@@ -125,6 +125,7 @@
 #include "net/ssl/ssl_config_service.h"
 #include "ppapi/buildflags/buildflags.h"
 #include "services/audio/public/cpp/audio_system_factory.h"
+#include "services/audio/public/mojom/constants.mojom.h"
 #include "services/resource_coordinator/public/cpp/memory_instrumentation/client_process_impl.h"
 #include "services/resource_coordinator/public/mojom/memory_instrumentation/memory_instrumentation.mojom.h"
 #include "services/resource_coordinator/public/mojom/service_constants.mojom.h"
@@ -1298,7 +1299,7 @@ int BrowserMainLoop::BrowserThreadsStarted() {
 
   {
     TRACE_EVENT0("startup", "BrowserThreadsStarted::Subsystem:AudioMan");
-    CreateAudioManager();
+    InitializeAudio();
   }
 
   {
@@ -1629,7 +1630,7 @@ void BrowserMainLoop::EndStartupTracing() {
           base::Bind(OnStoppedStartupTracing, startup_trace_file_)));
 }
 
-void BrowserMainLoop::CreateAudioManager() {
+void BrowserMainLoop::InitializeAudio() {
   DCHECK(!audio_manager_);
 
   audio_manager_ = GetContentClient()->browser()->CreateAudioManager(
@@ -1660,6 +1661,12 @@ void BrowserMainLoop::CreateAudioManager() {
     audio_service_runner_->StartWithTaskRunner(audio_manager_->GetTaskRunner());
   }
 
+  if (base::FeatureList::IsEnabled(features::kAudioServiceLaunchOnStartup)) {
+    content::ServiceManagerConnection::GetForProcess()
+        ->GetConnector()
+        ->StartService(audio::mojom::kServiceName);
+  }
+
   audio_system_ = audio::CreateAudioSystem(
       content::ServiceManagerConnection::GetForProcess()
           ->GetConnector()

content/browser/browser_main_loop.h

@@ -254,7 +254,7 @@ class CONTENT_EXPORT BrowserMainLoop {
   void InitStartupTracingForDuration();
   void EndStartupTracing();
 
-  void CreateAudioManager();
+  void InitializeAudio();
 
   bool UsingInProcessGpu() const;
 

content/public/common/content_features.cc

@@ -36,6 +36,10 @@ const base::Feature kAsyncWheelEvents{"AsyncWheelEvents",
 const base::Feature kAudioServiceAudioStreams{
     "AudioServiceAudioStreams", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Launches the audio service on the browser startup.
+const base::Feature kAudioServiceLaunchOnStartup{
+    "AudioServiceLaunchOnStartup", base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Runs the audio service in a separate process.
 const base::Feature kAudioServiceOutOfProcess{
     "AudioServiceOutOfProcess", base::FEATURE_DISABLED_BY_DEFAULT};

content/public/common/content_features.h

@@ -22,6 +22,7 @@ CONTENT_EXPORT extern const base::Feature
 CONTENT_EXPORT extern const base::Feature kAsmJsToWebAssembly;
 CONTENT_EXPORT extern const base::Feature kAsyncWheelEvents;
 CONTENT_EXPORT extern const base::Feature kAudioServiceAudioStreams;
+CONTENT_EXPORT extern const base::Feature kAudioServiceLaunchOnStartup;
 CONTENT_EXPORT extern const base::Feature kAudioServiceOutOfProcess;
 CONTENT_EXPORT extern const base::Feature kBlinkHeapIncrementalMarking;
 CONTENT_EXPORT extern const base::Feature kBlockCredentialedSubresources;

services/audio/service.cc

@@ -63,6 +63,9 @@ void Service::OnStart() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   TRACE_EVENT0("audio", "audio::Service::OnStart")
 
+  // This will pre-create AudioManager if AudioManagerAccessor owns it.
+  CHECK(audio_manager_accessor_->GetAudioManager());
+
   metrics_ =
       std::make_unique<ServiceMetrics>(base::DefaultTickClock::GetInstance());
   ref_factory_ = std::make_unique<service_manager::ServiceContextRefFactory>(