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);

[rohitrao (ping after 24h), 2017/01/23 20:31:39]

What's the relationship between DesktopPromotionSyncObserver and BrowserState? 
Is it 1:1?  This constructor implies that it is 1:1, but other code makes it
seem like there is no observer for the corresponding incognito BrowserState.  Is
there only ever one DesktopPromotionSyncObserver per app?

(Another way to think about this -- if we added multi-profile support to iOS, so
that two users could run chrome with different BrowserStates and different
profile directories, would we have two DesktopPromotionSyncObservers or still
only one?)

[mrefaat, 2017/01/24 19:57:20]

i think it's 1:1 if we ever have more than 1 profile we will need observer for
each one. as the sync state is profile related.

+  ~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_