[Sync] ModelTypeStore factory shouldn't require valid PSS to function correctly

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 92250fdffc2c57c80135401bbc9da60cbbfa00f5..741a52252772aaad304d5183ce87f2f886cf5977 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -176,6 +176,10 @@ void IOSChromeSyncClient::Initialize() {
   }
 }
 
+base::SequencedWorkerPool* IOSChromeSyncClient::GetBlockingPool() {
+  return web::WebThread::GetBlockingPool();
+}
+
 syncer::SyncService* IOSChromeSyncClient::GetSyncService() {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   return IOSChromeProfileSyncServiceFactory::GetForBrowserState(browser_state_);