Manage UsbService lifetime in DeviceClient implementations.

device/core/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 "device/core/device_client.h"
 
 #include "base/logging.h"
 
 namespace device {
 
 namespace {
 
-DeviceClient* g_instance = NULL;
+DeviceClient* g_instance;
 
 }  // namespace
 
 DeviceClient::DeviceClient() {
-  DCHECK(!g_instance);
   g_instance = this;
 }
 
 DeviceClient::~DeviceClient() {
-  g_instance = NULL;
+  g_instance = nullptr;
 }
 
 /* static */
 DeviceClient* DeviceClient::Get() {
   DCHECK(g_instance);
   return g_instance;
 }
 
 UsbService* DeviceClient::GetUsbService() {
   // This should never be called by clients which do not support the USB API.
   NOTREACHED();
-  return NULL;
+  return nullptr;
 }
 
 HidService* DeviceClient::GetHidService() {
   // This should never be called by clients which do not support the HID API.
   NOTREACHED();
-  return NULL;
+  return nullptr;
 }
 
 }  // namespace device