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

components/sync/engine_impl/sync_manager_impl.cc

Index: components/sync/engine_impl/sync_manager_impl.cc
diff --git a/components/sync/engine_impl/sync_manager_impl.cc b/components/sync/engine_impl/sync_manager_impl.cc
index f0b7cada4332de19b27870ea39136655e47e80f2..06720a0318cd330c99eae2c460fe70087c2c9989 100644
--- a/components/sync/engine_impl/sync_manager_impl.cc
+++ b/components/sync/engine_impl/sync_manager_impl.cc
@@ -47,6 +47,10 @@
 #include "components/sync/syncable/write_node.h"
 #include "components/sync/syncable/write_transaction.h"
 
+#if defined(OS_WIN)
+#include "components/sync/engine_impl/loopback_server/loopback_connection_manager.h"
+#endif
+
 using base::TimeDelta;
 using sync_pb::GetUpdatesCallerInfo;
 
@@ -282,11 +286,21 @@ void SyncManagerImpl::Init(InitArgs* args) {
     args->saved_nigori_state.reset();
   }
 
-  connection_manager_ = base::MakeUnique<SyncServerConnectionManager>(
-      args->service_url.host() + args->service_url.path(),
-      args->service_url.EffectiveIntPort(),
-      args->service_url.SchemeIsCryptographic(), args->post_factory.release(),
-      args->cancelation_signal);
+  if (args->enable_local_sync_backend) {
+#if defined(OS_WIN)
+    VLOG(1) << "Running against local sync backend.";
+    connection_manager_ = base::MakeUnique<LoopbackConnectionManager>(
+        args->cancelation_signal, args->local_sync_backend_folder);
+#else
+    NOTREACHED();
+#endif  // defined(OS_WIN)
+  } else {
+    connection_manager_ = base::MakeUnique<SyncServerConnectionManager>(
+        args->service_url.host() + args->service_url.path(),
+        args->service_url.EffectiveIntPort(),
+        args->service_url.SchemeIsCryptographic(), args->post_factory.release(),
+        args->cancelation_signal);
+  }
   connection_manager_->set_client_id(directory()->cache_guid());
   connection_manager_->AddListener(this);