services/ui: Allow injecting the IO thread task runner for gpu channel.

services/ui/public/cpp/gpu_service.cc

Index: services/ui/public/cpp/gpu_service.cc
diff --git a/services/ui/public/cpp/gpu_service.cc b/services/ui/public/cpp/gpu_service.cc
index 6c70440f940189eca61194151bf4c52152597284..71c973f1d9e1d1ea6674e3b721072a7262565bc3 100644
--- a/services/ui/public/cpp/gpu_service.cc
+++ b/services/ui/public/cpp/gpu_service.cc
@@ -35,15 +35,11 @@ GpuService::GpuService(shell::Connector* connector)
       connector_(connector),
       shutdown_event_(base::WaitableEvent::ResetPolicy::AUTOMATIC,
                       base::WaitableEvent::InitialState::NOT_SIGNALED),
-      io_thread_("GPUIOThread"),
       gpu_memory_buffer_manager_(new MojoGpuMemoryBufferManager),
       is_establishing_(false),
       establishing_condition_(&lock_) {
   DCHECK(main_task_runner_);
   DCHECK(connector_);
-  base::Thread::Options thread_options(base::MessageLoop::TYPE_IO, 0);
-  thread_options.priority = base::ThreadPriority::NORMAL;
-  CHECK(io_thread_.StartWithOptions(thread_options));
 }
 
 GpuService::~GpuService() {
@@ -58,6 +54,12 @@ std::unique_ptr<GpuService> GpuService::Initialize(
   return base::WrapUnique(new GpuService(connector));
 }
 
+void GpuService::SetIOThreadTaskRunner(
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
+  DCHECK(!io_task_runner_);
+  io_task_runner_ = std::move(task_runner);
+}
+
 void GpuService::EstablishGpuChannel(
     const gpu::GpuChannelEstablishedCallback& callback) {
   base::AutoLock auto_lock(lock_);
@@ -205,7 +207,14 @@ bool GpuService::IsMainThread() {
 
 scoped_refptr<base::SingleThreadTaskRunner>
 GpuService::GetIOThreadTaskRunner() {
-  return io_thread_.task_runner();
+  if (!io_task_runner_) {
+    io_thread_.reset(new base::Thread("GPUIOThread"));
+    base::Thread::Options thread_options(base::MessageLoop::TYPE_IO, 0);
+    thread_options.priority = base::ThreadPriority::NORMAL;
+    CHECK(io_thread_->StartWithOptions(thread_options));
+    io_task_runner_ = io_thread_->task_runner();
+  }
+  return io_task_runner_;
 }
 
 std::unique_ptr<base::SharedMemory> GpuService::AllocateSharedMemory(