Add worker thread to audio::OwningAudioManagerAccessor.

When the Audio Service will be running in its standalone process, the
worker thread will be used to drive the audio flows for the loopback
streams, as well as all the fake streams (including the those used for
muting). These things should not become blocked by long-running control
tasks on the main thread.

Bug: 824019
Change-Id: I6b7fd4cb110842c70a26072161dc91929ed4f19d
Reviewed-on: https://chromium-review.googlesource.com/1046153
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556369}

services/audio/owning_audio_manager_accessor.cc

@@ -4,8 +4,12 @@
 
 #include "services/audio/owning_audio_manager_accessor.h"
 
+#include <memory>
+#include <utility>
+
 #include "base/macros.h"
 #include "base/single_thread_task_runner.h"
+#include "base/threading/thread.h"
 #include "media/audio/audio_manager.h"
 #include "media/audio/audio_thread.h"
 
@@ -14,7 +18,8 @@ namespace audio {
 namespace {
 
 // Thread class for hosting owned AudioManager on the main thread of the
-// service.
+// service, with a separate worker thread (started on-demand) for running things
+// that shouldn't be blocked by main-thread tasks.
 class MainThread : public media::AudioThread {
  public:
   MainThread();
@@ -27,10 +32,17 @@ class MainThread : public media::AudioThread {
 
  private:
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
+  // This is not started until the first time GetWorkerTaskRunner() is called.
+  base::Thread worker_thread_;
+  scoped_refptr<base::SingleThreadTaskRunner> worker_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(MainThread);
 };
 
-MainThread::MainThread() : task_runner_(base::ThreadTaskRunnerHandle::Get()) {}
+MainThread::MainThread()
+    : task_runner_(base::ThreadTaskRunnerHandle::Get()),
+      worker_thread_("AudioWorkerThread") {}
 
 MainThread::~MainThread() {
   DCHECK(task_runner_->BelongsToCurrentThread());
@@ -38,6 +50,10 @@ MainThread::~MainThread() {
 
 void MainThread::Stop() {
   DCHECK(task_runner_->BelongsToCurrentThread());
+  if (worker_task_runner_) {
+    worker_task_runner_ = nullptr;
+    worker_thread_.Stop();
+  }
 }
 
 base::SingleThreadTaskRunner* MainThread::GetTaskRunner() {
@@ -45,8 +61,12 @@ base::SingleThreadTaskRunner* MainThread::GetTaskRunner() {
 }
 
 base::SingleThreadTaskRunner* MainThread::GetWorkerTaskRunner() {
-  NOTREACHED();
-  return task_runner_.get();
+  DCHECK(task_runner_->BelongsToCurrentThread());
+  if (!worker_task_runner_) {
+    CHECK(worker_thread_.Start());
+    worker_task_runner_ = worker_thread_.task_runner();
+  }
+  return worker_task_runner_.get();
 }
 
 }  // namespace