[Sync] Allow sync start without sign-in if the local sync backend is on.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index 8a9d6df46a7d1bc8d8010c034ed85f48b4b8c3a0..3c047ea3a957e617f7e5e2f0470e923de151f3e9 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -336,6 +336,20 @@ static_library("sync") {
     "engine_impl/js_sync_encryption_handler_observer.h",
     "engine_impl/js_sync_manager_observer.cc",
     "engine_impl/js_sync_manager_observer.h",
+    "engine_impl/loopback_server/loopback_connection_manager.cc",
+    "engine_impl/loopback_server/loopback_connection_manager.h",
+    "engine_impl/loopback_server/loopback_server.cc",
+    "engine_impl/loopback_server/loopback_server.h",
+    "engine_impl/loopback_server/loopback_server_entity.cc",
+    "engine_impl/loopback_server/loopback_server_entity.h",
+    "engine_impl/loopback_server/persistent_bookmark_entity.cc",
+    "engine_impl/loopback_server/persistent_bookmark_entity.h",
+    "engine_impl/loopback_server/persistent_permanent_entity.cc",
+    "engine_impl/loopback_server/persistent_permanent_entity.h",
+    "engine_impl/loopback_server/persistent_tombstone_entity.cc",
+    "engine_impl/loopback_server/persistent_tombstone_entity.h",
+    "engine_impl/loopback_server/persistent_unique_client_entity.cc",
+    "engine_impl/loopback_server/persistent_unique_client_entity.h",
     "engine_impl/model_type_connector_proxy.cc",
     "engine_impl/model_type_connector_proxy.h",
     "engine_impl/model_type_registry.cc",
@@ -607,25 +621,6 @@ static_library("sync") {
     ]
   }
 
-  if (is_win) {
-    sources += [
-      "engine_impl/loopback_server/loopback_connection_manager.cc",
-      "engine_impl/loopback_server/loopback_connection_manager.h",
-      "engine_impl/loopback_server/loopback_server.cc",
-      "engine_impl/loopback_server/loopback_server.h",
-      "engine_impl/loopback_server/loopback_server_entity.cc",
-      "engine_impl/loopback_server/loopback_server_entity.h",
-      "engine_impl/loopback_server/persistent_bookmark_entity.cc",
-      "engine_impl/loopback_server/persistent_bookmark_entity.h",
-      "engine_impl/loopback_server/persistent_permanent_entity.cc",
-      "engine_impl/loopback_server/persistent_permanent_entity.h",
-      "engine_impl/loopback_server/persistent_tombstone_entity.cc",
-      "engine_impl/loopback_server/persistent_tombstone_entity.h",
-      "engine_impl/loopback_server/persistent_unique_client_entity.cc",
-      "engine_impl/loopback_server/persistent_unique_client_entity.h",
-    ]
-  }
-
   if (!is_ios) {
     sources += [
       "driver/sync_policy_handler.cc",