Remove dependency on BrowserThread::FILE from Sync code

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 d066897389a38fff3855629d0161ffba49403250..ed4d4487a41615a19d07e876705c165d5c4d6feb 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -383,9 +383,8 @@ IOSChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
           web::WebThread::GetTaskRunnerForThread(web::WebThread::DB),
           syncer::GROUP_DB);
     case syncer::GROUP_FILE:
-      return new syncer::SequencedModelWorker(
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE),
-          syncer::GROUP_FILE);
+      // Not supported on iOS.
+      return nullptr;
     case syncer::GROUP_UI:
       return new syncer::UIModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));