Use the task scheduler for blocking tasks in the USB service on Linux

device/usb/usb_service.h

Index: device/usb/usb_service.h
diff --git a/device/usb/usb_service.h b/device/usb/usb_service.h
index 15977dd1c544a5270084dc9a0a3eb57609b70c11..6865ca4ebbc5f8e955464ee205c9f378f3549da8 100644
--- a/device/usb/usb_service.h
+++ b/device/usb/usb_service.h
@@ -54,6 +54,9 @@ class UsbService : public base::NonThreadSafe {
   static std::unique_ptr<UsbService> Create(
       scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
 
+  // Creates a SequencedTaskRunner appropriate for blocking I/O operations.
+  static scoped_refptr<base::SequencedTaskRunner> CreateBlockingTaskRunner();
+
   virtual ~UsbService();
 
   scoped_refptr<UsbDevice> GetDevice(const std::string& guid);