[Sync] Convert NonBlockingDataTypeController to be single-thread.

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 9b17cbfa2fea7ba3ffff58637a5c19eb5cc06afe..0e6c753b1c060dd7fa335bc0c2065360381df4a2 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -327,16 +327,17 @@ IOSChromeSyncClient::GetSyncableServiceForType(syncer::ModelType type) {
   }
 }
 
-syncer_v2::ModelTypeService* IOSChromeSyncClient::GetModelTypeServiceForType(
-    syncer::ModelType type) {
+base::WeakPtr<syncer_v2::ModelTypeService>
+IOSChromeSyncClient::GetModelTypeServiceForType(syncer::ModelType type) {
   switch (type) {
     case syncer::DEVICE_INFO:
       return IOSChromeProfileSyncServiceFactory::GetForBrowserState(
                  browser_state_)
-          ->GetDeviceInfoService();
+          ->GetDeviceInfoService()
+          ->AsWeakPtr();
     default:
       NOTREACHED();
-      return nullptr;
+      return base::WeakPtr<syncer_v2::ModelTypeService>();
   }
 }