[Sync] Use forwarding delegate for PRINTERS sync

Printers sync runs on the UI thread but so far uses a proxy delegate
that posts tasks from UI thread to UI thread.

This CL simplifies that and employs a forwarding delegate that runs
tasks synchronously. After this CL, all data types running on the UI
thread will have forwarding delegates.

Bug: 867801
Change-Id: I08e261323a9caa816899dd924f09ecdee69ec936
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621907
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661732}

components/browser_sync/profile_sync_components_factory_impl.cc

@@ -335,18 +335,8 @@ ProfileSyncComponentsFactoryImpl::CreateCommonDataTypeControllers(
 
 #if defined(OS_CHROMEOS)
   if (!disabled_types.Has(syncer::PRINTERS)) {
-    // TODO(crbug.com/867801) This still uses a proxy delegate even though the
-    // model lives on the UI thread. Switching to forwarding delegate causes
-    // cryptic test failures on chromeos. Fix that and switch to the forwarding
-    // delegate.
-    controllers.push_back(std::make_unique<ModelTypeController>(
-        syncer::PRINTERS,
-        std::make_unique<syncer::ProxyModelTypeControllerDelegate>(
-            ui_thread_,
-            base::BindRepeating(&browser_sync::BrowserSyncClient::
-                                    GetControllerDelegateForModelType,
-                                base::Unretained(sync_client_),
-                                syncer::PRINTERS))));
+    controllers.push_back(
+        CreateModelTypeControllerForModelRunningOnUIThread(syncer::PRINTERS));
   }
 #endif