Remove BrowserThread dependency from usb_service.

device/common/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 DEVICE_COMMON_DEVICE_CLIENT_H_
+#define DEVICE_COMMON_DEVICE_CLIENT_H_
+
+namespace usb_service {
+class UsbService;
+}
+
+namespace device {
+
+// Interface used by consumers of //device APIs to get pointers to the service
+// singletons appropriate for a given embedding application. For an example see
+// //chrome/browser/chrome_device_client.h.
+class DeviceClient {
+ public:
+  // Construction sets the single instance.
+  DeviceClient();
+
+  // Destruction clears the single instance.
+  ~DeviceClient();
+
+  // Returns the single instance of |this|.
+  static DeviceClient* Get();
+
+  // Returns the UsbService instance for this embedder.
+  virtual usb_service::UsbService* GetUsbService();
+};
+
+}  // namespace device
+
+#endif  // DEVICE_COMMON_DEVICE_CLIENT_H_