cros: Fix sync_integration_tests when SplitSettingsSync is enabled

SingleClientStandaloneTransportSyncTest needs to include WEB_APPS in
its list of OS transport-mode types.

sync_integration_tests --enable-features=SplitSettingsSync passes
after this change.

This CL is in preparation to turn SplitSettingsSync on by default.

Bug: 1060289
Change-Id: I7662ba9a4ac529db18ba8887eeebe6f4023a53f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211044
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771500}

chrome/browser/sync/test/integration/single_client_standalone_transport_sync_test.cc

@@ -47,7 +47,7 @@ syncer::ModelTypeSet AllowedTypesInStandaloneTransportMode() {
     allowed_types.PutAll({syncer::APPS, syncer::APP_SETTINGS, syncer::APP_LIST,
                           syncer::APP_SETTINGS, syncer::ARC_PACKAGE,
                           syncer::PRINTERS, syncer::OS_PREFERENCES,
-                          syncer::OS_PRIORITY_PREFERENCES});
+                          syncer::OS_PRIORITY_PREFERENCES, syncer::WEB_APPS});
   }
   if (base::FeatureList::IsEnabled(switches::kSyncWifiConfigurations)) {
     allowed_types.Put(syncer::WIFI_CONFIGURATIONS);