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

device/udev_linux/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 
 if (use_udev) {
   source_set("udev_linux") {
     sources = [
       "scoped_udev.h",
       "udev.cc",
       "udev.h",
       "udev0_loader.cc",
       "udev0_loader.h",
       "udev1_loader.cc",
       "udev1_loader.h",
       "udev_linux.cc",
       "udev_linux.h",
       "udev_loader.cc",
       "udev_loader.h",
+      "udev_watcher.cc",
+      "udev_watcher.h",
     ]
 
     deps = [
       "//base",
       "//build/linux/libudev",
     ]
   }
 }