Use FileDescriptorWatcher in UsbDeviceHandleUsbfs.

device/usb/usb_device_handle_usbfs.h

 // Copyright 2016 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_DEVICE_HANDLE_USBFS_H_
 #define DEVICE_USB_USB_DEVICE_HANDLE_USBFS_H_
 
 #include <list>
 #include <map>
 #include <memory>
 #include <vector>
 
 #include "base/files/scoped_file.h"
 #include "device/usb/usb_device_handle.h"
 
 struct usbdevfs_urb;
 
 namespace base {
 class SequencedTaskRunner;
 class SingleThreadTaskRunner;
 }
 
 namespace device {
 
 // Implementation of a USB device handle on top of the Linux USBFS ioctl
 // interface available on Linux, Chrome OS and Android.
 class UsbDeviceHandleUsbfs : public UsbDeviceHandle {
  public:
   // Constructs a new device handle from an existing |device| and open file
-  // descriptor to that device. |blocking_task_runner| must have a
-  // MessageLoopForIO.
+  // descriptor to that device. |blocking_task_runner| must run tasks on a
+  // thread that supports FileDescriptorWatcher.
   UsbDeviceHandleUsbfs(
       scoped_refptr<UsbDevice> device,
       base::ScopedFD fd,
       scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
 
   // UsbDeviceHandle implementation.
   scoped_refptr<UsbDevice> GetDevice() const override;
   void Close() override;
   void SetConfiguration(int configuration_value,
                         const ResultCallback& callback) override;
@@ skipping 120 matching lines @@
   // thread's message loop is destroyed but can also be freed by this class on
   // destruction.
   FileThreadHelper* helper_;
 
   std::list<std::unique_ptr<Transfer>> transfers_;
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_DEVICE_HANDLE_USBFS_H_