Use the task scheduler in the USB service on macOS and Windows

device/usb/usb_service_impl.cc

Index: device/usb/usb_service_impl.cc
diff --git a/device/usb/usb_service_impl.cc b/device/usb/usb_service_impl.cc
index 2408eb4b32e06af9ff2dc5f4e33116681fce8cbd..dba15d978a1564ddab092ba409003bb6a0e41f04 100644
--- a/device/usb/usb_service_impl.cc
+++ b/device/usb/usb_service_impl.cc
@@ -19,6 +19,7 @@
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/task_scheduler/post_task.h"
 #include "build/build_config.h"
 #include "components/device_event_log/device_event_log.h"
 #include "device/usb/usb_device_handle.h"
@@ -216,15 +217,19 @@ void OnDeviceOpenedReadDescriptors(
 
 }  // namespace
 
-UsbServiceImpl::UsbServiceImpl(
-    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_in)
-    : UsbService(std::move(blocking_task_runner_in)),
+UsbServiceImpl::UsbServiceImpl()
+    : UsbService(nullptr),
 #if defined(OS_WIN)
       device_observer_(this),
 #endif
       weak_factory_(this) {
-  blocking_task_runner()->PostTask(
+  base::PostTaskWithTraits(

[robliao, 2017/05/02 14:32:59]

Calls to this API are done in parallel. If this task needs to run before the the
one below, it may be more robust to have a sequenced blocking task runner here
(or if RefreshDevices is guaranteed to not be called before this is complete,
then it's all good here).

[Reilly Grant (use Gerrit), 2017/05/02 23:41:44]

That's fine. RefreshDevices is guaranteed not to post its task until this task
is done.

       FROM_HERE,
+      base::TaskTraits()
+          .MayBlock()
+          .WithPriority(base::TaskPriority::USER_VISIBLE)
+          .WithShutdownBehavior(
+              base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN),
       base::Bind(&InitializeUsbContextOnBlockingThread, task_runner(),
                  base::Bind(&UsbServiceImpl::OnUsbContext,
                             weak_factory_.GetWeakPtr())));
@@ -324,11 +329,17 @@ void UsbServiceImpl::RefreshDevices() {
     pending_path_enumerations_.pop();
   }
 
-  blocking_task_runner()->PostTask(
+  base::PostTaskWithTraits(
       FROM_HERE,
+      base::TaskTraits()
+          .MayBlock()
+          .WithPriority(base::TaskPriority::USER_VISIBLE)
+          .WithShutdownBehavior(
+              base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN),
       base::Bind(&GetDeviceListOnBlockingThread, device_path, context_,
-                 task_runner(), base::Bind(&UsbServiceImpl::OnDeviceList,
-                                           weak_factory_.GetWeakPtr())));
+                 task_runner(),
+                 base::Bind(&UsbServiceImpl::OnDeviceList,
+                            weak_factory_.GetWeakPtr())));
 }
 
 void UsbServiceImpl::OnDeviceList(libusb_device** platform_devices,
@@ -437,8 +448,8 @@ void UsbServiceImpl::EnumerateDevice(PlatformUsbDevice platform_device,
       return;
     }
 
-    scoped_refptr<UsbDeviceImpl> device(new UsbDeviceImpl(
-        context_, platform_device, descriptor, blocking_task_runner()));
+    scoped_refptr<UsbDeviceImpl> device(
+        new UsbDeviceImpl(context_, platform_device, descriptor));
     base::Closure add_device =
         base::Bind(&UsbServiceImpl::AddDevice, weak_factory_.GetWeakPtr(),
                    refresh_complete, device);