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

components/sync/engine_impl/loopback_server/loopback_server.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/engine_impl/loopback_server/loopback_server.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 #include <limits>
@@ skipping 505 matching lines @@
 
   return true;
 }
 
 // Saves all entities and server state to a protobuf file in |filename|.
 bool LoopbackServer::SaveStateToFile(const base::FilePath& filename) const {
   sync_pb::LoopbackServerProto proto;
   SerializeState(&proto);
 
   std::string serialized = proto.SerializeAsString();
+  CHECK(base::CreateDirectory(filename.DirName()));

[pavely, 2016/10/27 23:53:39]

Failure to create directory is the same as failure to write contents to file. In
both cases what's on disk doesn't match what the code thinks should be there. I
think you should treat them similarly, either both CHECK or both "return
false;".

[pastarmovj, 2016/11/02 13:39:25]

Agree and it should not crash for this reason imho - rather log and continue
working in local mode.

   int result = base::WriteFile(filename, serialized.data(), serialized.size());
   return result == static_cast<int>(serialized.size());
 }
 
 // Loads all entities and server state from a protobuf file in |filename|.
 bool LoopbackServer::LoadStateFromFile(const base::FilePath& filename) {
   if (base::PathExists(filename)) {
     std::string serialized;
     if (base::ReadFileToString(filename, &serialized)) {
       sync_pb::LoopbackServerProto proto;
@@ skipping 11 matching lines @@
       // write.
       LOG(ERROR) << "Loopback sync can not read the persistent state file.";
       return false;
     }
   }
   LOG(WARNING) << "Loopback sync persistent state file does not exist.";
   return false;
 }
 
 }  // namespace syncer