Use Sync FakeServer in exponential backoff tests

sync/test/fake_server/fake_server.h

 // 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.
 
 #ifndef SYNC_TEST_FAKE_SERVER_FAKE_SERVER_H_
 #define SYNC_TEST_FAKE_SERVER_FAKE_SERVER_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 76 matching lines @@
     const sync_pb::SyncEnums::Action& action);
 
   // Adds |observer| to FakeServer's observer list. This should be called
   // before the Profile associated with |observer| is connected to the server.
   void AddObserver(Observer* observer);
 
   // Removes |observer| from the FakeServer's observer list. This method
   // must be called if AddObserver was ever called with |observer|.
   void RemoveObserver(Observer* observer);
 
+  // Undoes the effects of DisableNetwork.
+  void EnableNetwork();
+
+  // Forces every request to fail in a way that simulates a network failure.
+  // This can be used to trigger exponential backoff in the client.
+  void DisableNetwork();
+
  private:
   typedef std::map<std::string, FakeServerEntity*> EntityMap;
 
   // Processes a GetUpdates call.
   bool HandleGetUpdatesRequest(const sync_pb::GetUpdatesMessage& get_updates,
                                sync_pb::GetUpdatesResponse* response);
 
   // Processes a Commit call.
   bool HandleCommitRequest(const sync_pb::CommitMessage& message,
                            const std::string& invalidator_client_id,
@@ skipping 54 matching lines @@
   // Used as the error_code field of ClientToServerResponse on all responses
   // except when |triggered_actionable_error_| is set.
   sync_pb::SyncEnums::ErrorType error_type_;
 
   // Used as the error field of ClientToServerResponse when its pointer is not
   // NULL.
   scoped_ptr<sync_pb::ClientToServerResponse_Error> triggered_actionable_error_;
 
   // FakeServer's observers.
   ObserverList<Observer, true> observers_;
+
+  // When true, the server operates normally. When false, a failure is returned
+  // on every request. This is used to simulate a network failure on the client.
+  bool network_enabled_;
 };
 
 }  // namespace fake_server
 
 #endif  // SYNC_TEST_FAKE_SERVER_FAKE_SERVER_H_