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

device/usb/usb_service.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ skipping 36 matching lines @@
 
     // Notifies the observer that the UsbService it depends on is shutting down.
     virtual void WillDestroyUsbService();
   };
 
   // The file task runner reference is used for blocking I/O operations.
   // Returns nullptr when initialization fails.
   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);
 
   // Shuts down the UsbService. Must be called before destroying the UsbService
   // when tasks can still be posted to the |blocking_task_runner| provided to
   // Create().
   virtual void Shutdown();
 
   // Enumerates available devices.
@@ skipping 36 matching lines @@
 #if DCHECK_IS_ON()
   bool did_shutdown_ = false;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_