Add Desktop iOS promotion logging to chrome ios app.

ios/chrome/browser/desktop_promotion/desktop_promotion_sync_service.h

+// 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.
+
+#ifndef CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_SYNC_SERVICE_H
+#define CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_SYNC_SERVICE_H
+
+#include "base/macros.h"
+#include "components/browser_sync/profile_sync_service.h"
+#include "ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.h"
+
+namespace user_prefs {
+class PrefRegistrySyncable;
+}
+
+// This class is responsible for creating a DesktopPromotionSyncObserver
+// after the main browser state is initialized.
+// An object from this class should only be created by
+// DesktopPromotionSyncServiceFactory.
+class DesktopPromotionSyncService : public KeyedService {
+ public:
+  // Only the DesktopPromotionSyncServiceFactory and tests should call this.
+  DesktopPromotionSyncService(PrefService* pref_service,
+                              browser_sync::ProfileSyncService* sync_service);
+
+  ~DesktopPromotionSyncService() override;
+
+  // Register profile specific desktop promotion related preferences.
+  static void RegisterDesktopPromotionUserPrefs(
+      user_prefs::PrefRegistrySyncable* registry);
+
+ private:
+  DesktopPromotionSyncObserver observer_;
+};
+
+#endif  // CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_SYNC_SERVICE_H