cros: Pull TestLockScreenClient into a separate MockLockScreenClient file.

ash/login/mock_lock_screen_client.cc

+// 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.
+
+#include "ash/login/mock_lock_screen_client.h"
+
+#include "ash/login/lock_screen_controller.h"
+#include "ash/shell.h"
+
+namespace ash {
+
+MockLockScreenClient::MockLockScreenClient() : binding_(this) {}
+
+MockLockScreenClient::~MockLockScreenClient() = default;
+
+mojom::LockScreenClientPtr MockLockScreenClient::CreateInterfacePtrAndBind() {
+  return binding_.CreateInterfacePtrAndBind();
+}
+
+std::unique_ptr<MockLockScreenClient> BindMockLockScreenClient() {
+  LockScreenController* lock_screen_controller =
+      Shell::Get()->lock_screen_controller();
+  auto lock_screen_client = base::MakeUnique<MockLockScreenClient>();
+  lock_screen_controller->SetClient(
+      lock_screen_client->CreateInterfacePtrAndBind());
+  return lock_screen_client;
+}
+
+}  // namespace ash