[sync] WebContentsObserver based sessions notifications

ios/chrome/browser/sync/ios_chrome_sync_client.mm

Index: ios/chrome/browser/sync/ios_chrome_sync_client.mm
diff --git a/ios/chrome/browser/sync/ios_chrome_sync_client.mm b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
index 741a52252772aaad304d5183ce87f2f886cf5977..ce78db87c458f7dd3eba7e3f0a2e9e946ccaab21 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -117,19 +117,22 @@ class SyncSessionsClientImpl : public sync_sessions::SyncSessionsClient {
     return window_delegates_getter_.get();
   }
 
-  std::unique_ptr<sync_sessions::LocalSessionEventRouter>
-  GetLocalSessionEventRouter() override {
+  sync_sessions::LocalSessionEventRouter* GetLocalSessionEventRouter()
+      override {
     syncer::SyncableService::StartSyncFlare flare(
         ios::sync_start_util::GetFlareForSyncableService(
             browser_state_->GetStatePath()));
-    return base::MakeUnique<IOSChromeLocalSessionEventRouter>(browser_state_,
-                                                              this, flare);
+    local_session_event_router_ =
+        base::MakeUnique<IOSChromeLocalSessionEventRouter>(browser_state_, this,
+                                                           flare);
+    return local_session_event_router_.get();
   }
 
  private:
   ios::ChromeBrowserState* const browser_state_;
   const std::unique_ptr<sync_sessions::SyncedWindowDelegatesGetter>
       window_delegates_getter_;
+  std::unique_ptr<IOSChromeLocalSessionEventRouter> local_session_event_router_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncSessionsClientImpl);
 };