Remove BrowserThread dependency from usb_service.

extensions/shell/browser/shell_device_client.cc

+// 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.
+
+#include "extensions/shell/browser/shell_device_client.h"
+
+#include "base/logging.h"
+#include "components/usb_service/usb_service.h"
+#include "content/public/browser/browser_thread.h"
+
+namespace extensions {
+
+ShellDeviceClient::ShellDeviceClient() {}
+
+ShellDeviceClient::~ShellDeviceClient() {}
+
+usb_service::UsbService* ShellDeviceClient::GetUsbService() {
+  return usb_service::UsbService::GetInstance(
+      content::BrowserThread::GetMessageLoopProxyForThread(
+          content::BrowserThread::UI));
+}
+
+}