Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/browser/ui/settings/personal_data_manager_data_changed_observer.cc

+// Copyright 2016 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/browser/ui/settings/personal_data_manager_data_changed_observer.h"
+
+#include "components/autofill/core/browser/personal_data_manager.h"
+
+PersonalDataManagerDataChangedObserver::PersonalDataManagerDataChangedObserver(
+    autofill::PersonalDataManager* personal_data_manager)
+    : personal_data_manager_(personal_data_manager) {
+  personal_data_manager_->AddObserver(this);
+}
+
+PersonalDataManagerDataChangedObserver::
+    ~PersonalDataManagerDataChangedObserver() {
+  personal_data_manager_->RemoveObserver(this);
+}
+
+void PersonalDataManagerDataChangedObserver::Wait() {
+  // If a test is blocked in that method, it means that OnPersonalDataChanged()
+  // was never called which indicates a bug in either the expectation of the
+  // test (no asynchronous operation is executed on the PersonalDataManager
+  // passed in the constructor) or in the utilisation of the
+  // PersonalDataManagerDataChangedObserver (there is a race-condition between
+  // sending the asynchronous operation and the creation of the observer, it can
+  // be fixed by creating the observer before sending the operation).
+  run_loop_.Run();
+}
+
+void PersonalDataManagerDataChangedObserver::OnPersonalDataChanged() {
+  run_loop_.QuitWhenIdle();
+}