[Sync] Adds a new switch for enabling the new local server backend.

components/sync/engine_impl/loopback_server/loopback_server.cc

Index: components/sync/engine_impl/loopback_server/loopback_server.cc
diff --git a/components/sync/engine_impl/loopback_server/loopback_server.cc b/components/sync/engine_impl/loopback_server/loopback_server.cc
index ef701df388764d1e214a5a27655260a578f109eb..88eb7ca095bbca0a13f7be92539401af5f6ad69f 100644
--- a/components/sync/engine_impl/loopback_server/loopback_server.cc
+++ b/components/sync/engine_impl/loopback_server/loopback_server.cc
@@ -523,6 +523,10 @@ bool LoopbackServer::SaveStateToFile(const base::FilePath& filename) const {
   SerializeState(&proto);
 
   std::string serialized = proto.SerializeAsString();
+  if (!base::CreateDirectory(filename.DirName())) {
+    LOG(ERROR) << "Loopback sync could not create the storage directory.";
+    return false;
+  }
   int result = base::WriteFile(filename, serialized.data(), serialized.size());
   return result == static_cast<int>(serialized.size());
 }