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

device/udev_linux/udev_watcher.h

Index: device/udev_linux/udev_watcher.h
diff --git a/device/udev_linux/udev_watcher.h b/device/udev_linux/udev_watcher.h
new file mode 100644
index 0000000000000000000000000000000000000000..0da372b291df67dcd0c7393575972496f88cb813
--- /dev/null
+++ b/device/udev_linux/udev_watcher.h
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+// Copyright 2017 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_UDEV_LINUX_UDEV_WATCHER_H_
+#define DEVICE_UDEV_LINUX_UDEV_WATCHER_H_
+
+#include <memory>
+
+#include "base/files/file_descriptor_watcher_posix.h"
+#include "base/macros.h"
+#include "base/sequence_checker.h"
+#include "device/udev_linux/scoped_udev.h"
+
+namespace device {
+
+// This class wraps an instance of udev_monitor, watching for devices that are
+// added and removed from the system. This class has sequence affinity.
+class UdevWatcher {
+ public:
+  class Observer {
+   public:
+    virtual ~Observer();
+    virtual void OnDeviceAdded(ScopedUdevDevicePtr device);
+    virtual void OnDeviceRemoved(ScopedUdevDevicePtr device);
+  };
+
+  static std::unique_ptr<UdevWatcher> StartWatching(Observer* observer);
+
+  ~UdevWatcher();
+
+  // Synchronously enumerates the all devices known to udev, calling
+  // OnDeviceAdded on the provided Observer for each.
+  void EnumerateExistingDevices();
+
+ private:
+  UdevWatcher(ScopedUdevPtr udev,
+              ScopedUdevMonitorPtr udev_monitor,
+              int monitor_fd,
+              Observer* observer);
+
+  void OnMonitorReadable();
+
+  ScopedUdevPtr udev_;
+  ScopedUdevMonitorPtr udev_monitor_;
+  Observer* observer_;
+  std::unique_ptr<base::FileDescriptorWatcher::Controller> file_watcher_;
+  base::SequenceChecker sequence_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(UdevWatcher);
+};
+
+}  // namespace device
+
+#endif  // DEVICE_UDEV_LINUX_UDEV_WATCHER_H_