[Sync] Add a sanity integration test for USS.

chrome/browser/sync/profile_sync_service_factory.cc

Index: chrome/browser/sync/profile_sync_service_factory.cc
diff --git a/chrome/browser/sync/profile_sync_service_factory.cc b/chrome/browser/sync/profile_sync_service_factory.cc
index 2c0ae010c6dbad6b892f1141c8fb1f78a2fb486c..a4322a7770465bb460a44197f77e7c422f2ca8cb 100644
--- a/chrome/browser/sync/profile_sync_service_factory.cc
+++ b/chrome/browser/sync/profile_sync_service_factory.cc
@@ -170,8 +170,12 @@ KeyedService* ProfileSyncServiceFactory::BuildServiceInstanceFor(
                                    ? ProfileSyncService::AUTO_START
                                    : ProfileSyncService::MANUAL_START;
 
-  init_params.sync_client =
-      base::MakeUnique<browser_sync::ChromeSyncClient>(profile);
+  if (!client_factory_) {
+    init_params.sync_client =
+        base::MakeUnique<browser_sync::ChromeSyncClient>(profile);
+  } else {
+    init_params.sync_client = client_factory_->Run(profile);
+  }
 
   init_params.network_time_update_callback = base::Bind(&UpdateNetworkTime);
   init_params.base_directory = profile->GetPath();
@@ -196,3 +200,13 @@ KeyedService* ProfileSyncServiceFactory::BuildServiceInstanceFor(
 bool ProfileSyncServiceFactory::HasProfileSyncService(Profile* profile) {
   return GetInstance()->GetServiceForBrowserContext(profile, false) != NULL;
 }
+
+// static
+void ProfileSyncServiceFactory::SetSyncClientFactoryForTest(
+    SyncClientFactory* client_factory) {
+  client_factory_ = client_factory;
+}
+
+// static
+ProfileSyncServiceFactory::SyncClientFactory*
+    ProfileSyncServiceFactory::client_factory_ = nullptr;