Add Desktop iOS promotion logging to chrome ios app.

ios/chrome/app/main_controller.mm

Index: ios/chrome/app/main_controller.mm
diff --git a/ios/chrome/app/main_controller.mm b/ios/chrome/app/main_controller.mm
index 291afaf25c157b84ec53d43c90e14adf54c44158..22b614e0756284f5a15c5c6da5fde8cab28ca223 100644
--- a/ios/chrome/app/main_controller.mm
+++ b/ios/chrome/app/main_controller.mm
@@ -71,6 +71,7 @@
 #include "ios/chrome/browser/crash_report/breakpad_helper.h"
 #import "ios/chrome/browser/crash_report/crash_report_background_uploader.h"
 #import "ios/chrome/browser/crash_report/crash_restore_helper.h"
+#include "ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.h"
 #include "ios/chrome/browser/experimental_flags.h"
 #include "ios/chrome/browser/file_metadata_util.h"
 #import "ios/chrome/browser/first_run/first_run.h"
@@ -335,6 +336,9 @@ enum class StackViewDismissalMode { NONE, NORMAL, INCOGNITO };
   // Bridge to listen to pref changes.
   std::unique_ptr<PrefObserverBridge> _localStatePrefObserverBridge;
 
+  // Sync service observer for the desktop ios promotion logging.
+  std::unique_ptr<DesktopPromotionSyncObserver> _desktopPromotionSyncObserver;
+
   // Registrar for pref changes notifications to the local state.
   PrefChangeRegistrar _localStatePrefChangeRegistrar;
 
@@ -1059,6 +1063,11 @@ enum class StackViewDismissalMode { NONE, NORMAL, INCOGNITO };
                         prefs::kMetricsReportingWifiOnly,
                         &_localStatePrefChangeRegistrar);
 
+                    // Track Sync state changes.
+                    _desktopPromotionSyncObserver.reset(
+                        new DesktopPromotionSyncObserver(
+                            [self currentBrowserState]));
+
                     // Calls the onPreferenceChanged function in case there was
                     // a
                     // change to the observed preferences before the observer