BookmarkSyncServiceFactory shouldn't create the BookmarkUndoService

Since the BookmarkSyncServiceFactory doesn't start the
BookmarkUndoService, it should use it only if it exists.

This was the case already in non-ios factory.
https://cs.chromium.org/chromium/src/chrome/browser/sync/bookmark_sync_service_factory.cc?l=38&rcl=46520ae13478333fe882e79041b667e5a3112796

However, by mistake, it was created in iOS factory
which caused tests to fail here:
https://chromium-review.googlesource.com/c/chromium/src/+/1052767

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5f205fa575773b3382b1e4c90ac4afa19d41d77a
Reviewed-on: https://chromium-review.googlesource.com/1171231Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582558}

ios/chrome/browser/bookmarks/bookmark_sync_service_factory.cc

@@ -41,7 +41,8 @@ BookmarkSyncServiceFactory::BuildServiceInstanceFor(
       ios::ChromeBrowserState::FromBrowserState(context);
   std::unique_ptr<sync_bookmarks::BookmarkSyncService> bookmark_sync_service(
       new sync_bookmarks::BookmarkSyncService(
-          BookmarkUndoServiceFactory::GetForBrowserState(browser_state)));
+          BookmarkUndoServiceFactory::GetForBrowserStateIfExists(
+              browser_state)));
   return bookmark_sync_service;
 }