Use Sync FakeServer in exponential backoff tests

sync/test/fake_server/fake_server.cc

 // Copyright 2014 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 "sync/test/fake_server/fake_server.h"
 
 #include <algorithm>
 #include <limits>
 #include <string>
 #include <vector>
@@ skipping 131 matching lines @@
 
   return scoped_ptr<UpdateSieve>(
       new UpdateSieve(request_from_version, min_version));
 }
 
 }  // namespace
 
 FakeServer::FakeServer() : version_(0),
                            store_birthday_(kDefaultStoreBirthday),
                            authenticated_(true),
-                           error_type_(sync_pb::SyncEnums::SUCCESS) {
+                           error_type_(sync_pb::SyncEnums::SUCCESS),
+                           network_enabled_(true) {
   keystore_keys_.push_back(kDefaultKeystoreKey);
   CHECK(CreateDefaultPermanentItems());
 }
 
 FakeServer::~FakeServer() {
   STLDeleteContainerPairSecondPointers(entities_.begin(), entities_.end());
 }
 
 bool FakeServer::CreateDefaultPermanentItems() {
   ModelTypeSet all_types = syncer::ProtocolTypes();
@@ skipping 48 matching lines @@
 }
 
 void FakeServer::SaveEntity(FakeServerEntity* entity) {
   delete entities_[entity->GetId()];
   entity->SetVersion(++version_);
   entities_[entity->GetId()] = entity;
 }
 
 void FakeServer::HandleCommand(const string& request,
                                const HandleCommandCallback& callback) {
+  if (!network_enabled_) {
+    callback.Run(net::ERR_FAILED, net::ERR_FAILED, string());
+    return;
+  }
+
   if (!authenticated_) {
     callback.Run(0, net::HTTP_UNAUTHORIZED, string());
     return;
   }
 
   sync_pb::ClientToServerMessage message;
   bool parsed = message.ParseFromString(request);
   CHECK(parsed) << "Unable to parse the ClientToServerMessage.";
 
   sync_pb::ClientToServerResponse response_proto;
@@ skipping 301 matching lines @@
 }
 
 void FakeServer::AddObserver(Observer* observer) {
   observers_.AddObserver(observer);
 }
 
 void FakeServer::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
 }
 
+void FakeServer::EnableNetwork() {
+  network_enabled_ = true;
+}
+
+void FakeServer::DisableNetwork() {
+  network_enabled_ = false;
+}
+
 }  // namespace fake_server