Simplify registration of a test ChromeBrowserStateManager.

ios/chrome/test/ios_chrome_scoped_testing_chrome_browser_state_manager.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 "ios/chrome/test/ios_chrome_scoped_testing_chrome_browser_state_manager.h"
+
+#include "ios/chrome/browser/application_context.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state_manager.h"
+#include "ios/chrome/test/testing_application_context.h"
+
+IOSChromeScopedTestingChromeBrowserStateManager::
+    IOSChromeScopedTestingChromeBrowserStateManager(
+        std::unique_ptr<ios::ChromeBrowserStateManager> browser_state_manager)
+    : browser_state_manager_(std::move(browser_state_manager)),
+      saved_browser_state_manager_(
+          GetApplicationContext()->GetChromeBrowserStateManager()) {
+  TestingApplicationContext::GetGlobal()->SetChromeBrowserStateManager(
+      browser_state_manager_.get());
+}
+
+IOSChromeScopedTestingChromeBrowserStateManager::
+    ~IOSChromeScopedTestingChromeBrowserStateManager() {
+  TestingApplicationContext::GetGlobal()->SetChromeBrowserStateManager(
+      saved_browser_state_manager_);
+}