[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..4e87763b591f7949e753af1664d14acd9acc1cfb 100644
--- a/components/sync/engine_impl/loopback_server/loopback_server.cc
+++ b/components/sync/engine_impl/loopback_server/loopback_server.cc
@@ -523,6 +523,7 @@ bool LoopbackServer::SaveStateToFile(const base::FilePath& filename) const {
   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());
 }