Add Desktop iOS promotion logging to chrome ios app.

ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.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 IOS_CHROME_BROWSER_SIGNIN_DESKTOP_PROMOTION_SYNC_OBSERVER_H_
+#define IOS_CHROME_BROWSER_SIGNIN_DESKTOP_PROMOTION_SYNC_OBSERVER_H_
+
+#include "base/macros.h"
+#include "components/sync/driver/sync_service_observer.h"
+
+namespace browser_sync {
+class ProfileSyncService;
+}
+
+class PrefService;
+
+// This class will implement ProfileSyncServiceObserver and will attach it self
+// to the sync service to observe the sync state change.
+// Once the sync state is changed and priority prefs are synced, the observer
+// will check the desktop promotion prefs, and if eligilble it will log desktop
+// promotion metrics to uma and mark the promotion cycle as completed in a perf.
+class DesktopPromotionSyncObserver : public syncer::SyncServiceObserver {
+ public:
+  DesktopPromotionSyncObserver(PrefService* pref_service,
+                               browser_sync::ProfileSyncService* sync_service);
+
+  ~DesktopPromotionSyncObserver() override;
+
+  // ProfileSyncServiceObserver implementation.
+  void OnStateChanged() override;
+
+ private:
+  PrefService* pref_service_;
+  browser_sync::ProfileSyncService* sync_service_;
+  bool desktop_metrics_logger_initiated_;
+
+  DISALLOW_COPY_AND_ASSIGN(DesktopPromotionSyncObserver);
+};
+
+#endif  // IOS_CHROME_BROWSER_SIGNIN_DESKTOP_PROMOTION_SYNC_OBSERVER_H_