[Sync] Return nullptr for OpenTabsUIDelegate when PROXY_TABS is disabled.

components/browser_sync/profile_sync_service.cc

 // Copyright (c) 2012 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.
 
 #include "components/browser_sync/profile_sync_service.h"
 
 #include <stddef.h>
 
 #include <cstddef>
 #include <map>
@@ skipping 406 matching lines @@
     syncer::ModelType type) const {
   DataTypeController::TypeMap::const_iterator iter =
       data_type_controllers_.find(type);
   if (iter == data_type_controllers_.end()) {
     return false;
   }
   return iter->second->state() == DataTypeController::RUNNING;
 }
 
 sync_sessions::OpenTabsUIDelegate* ProfileSyncService::GetOpenTabsUIDelegate() {
-  if (!IsDataTypeControllerRunning(syncer::SESSIONS))
-    return nullptr;
-  return sessions_sync_manager_.get();
+  // Although the backing data actually is of type |SESSIONS|, the desire to use
+  // open tabs functionality is tracked by the state of the |PROXY_TABS| type.
+  return IsDataTypeControllerRunning(syncer::PROXY_TABS)
+             ? sessions_sync_manager_.get()
+             : nullptr;
 }
 
 sync_sessions::FaviconCache* ProfileSyncService::GetFaviconCache() {
   return sessions_sync_manager_->GetFaviconCache();
 }
 
 syncer::DeviceInfoTracker* ProfileSyncService::GetDeviceInfoTracker() const {
   // One of the two should always be non-null after initialization is done.
   if (device_info_service_) {
     return device_info_service_.get();
@@ skipping 2085 matching lines @@
 
   DCHECK(startup_controller_->IsSetupInProgress());
   startup_controller_->SetSetupInProgress(false);
 
   if (IsBackendInitialized())
     ReconfigureDatatypeManager();
   NotifyObservers();
 }
 
 }  // namespace browser_sync