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_LOGGER_H_
+#define IOS_CHROME_BROWSER_SIGNIN_DESKTOP_PROMOTION_SYNC_OBSERVER_LOGGER_H_
+
+#include "base/macros.h"
+#include "components/sync/driver/sync_service_observer.h"
+
+namespace ios {
+class ChromeBrowserState;
+}
+
+class DesktopPromotionSyncObserver : public syncer::SyncServiceObserver {
+ public:
+  explicit DesktopPromotionSyncObserver(ios::ChromeBrowserState* browser_state);
+  ~DesktopPromotionSyncObserver() override;
+  // ProfileSyncServiceObserver implementation.
+  void OnStateChanged() override;
+
+ private:
+  ios::ChromeBrowserState* browser_state_;  // weak
+  bool desktop_metrics_logger_initiated_;
+
+  DISALLOW_COPY_AND_ASSIGN(DesktopPromotionSyncObserver);
+};
+
+#endif  // IOS_CHROME_BROWSER_SIGNIN_DESKTOP_PROMOTION_SYNC_OBSERVER_H_