Remove BrowserThread dependency from usb_service.

extensions/shell/browser/shell_device_client.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 EXTENSIONS_SHELL_BROWSER_SHELL_DEVICE_CLIENT_H_
+#define EXTENSIONS_SHELL_BROWSER_SHELL_DEVICE_CLIENT_H_
+
+#include "device/core/device_client.h"
+
+#include "base/compiler_specific.h"
+#include "base/macros.h"
+
+namespace extensions {
+
+// Implementation of device::DeviceClient that returns //device API service
+// singletons appropriate for use within app shell.
+class ShellDeviceClient : device::DeviceClient {
+ public:
+  ShellDeviceClient();
+  virtual ~ShellDeviceClient();
+
+  // device::DeviceClient implementation
+  virtual usb_service::UsbService* GetUsbService() OVERRIDE;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ShellDeviceClient);
+};
+
+}
+
+#endif  // EXTENSIONS_SHELL_BROWSER_SHELL_DEVICE_CLIENT_H_