[Sync] Remove ModelError::IsSet() in favor of base::Optional.

components/sync/model/model_error.h

 // Copyright 2017 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 COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_
 #define COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/location.h"
 
 namespace syncer {
 
 // A minimal error object for use by USS model type code.
 class ModelError {
  public:
-  // Creates an un-set error object (indicating an operation was successful).
-  ModelError();
-
   // Creates a set error object with the given location and message.
   ModelError(const tracked_objects::Location& location,
              const std::string& message);
 
   ~ModelError();
 
-  // Whether this object represents an actual error.
-  bool IsSet() const;
-
   // The location of the error this object represents. Can only be called if the
   // error is set.
   const tracked_objects::Location& location() const;
 
   // The message explaining the error this object represents. Can only be called
   // if the error is set.
   const std::string& message() const;
 
  private:
-  bool is_set_;
   tracked_objects::Location location_;
   std::string message_;
 };
 
 // Typedef for a simple error handler callback.
 using ModelErrorHandler = base::RepeatingCallback<void(const ModelError&)>;
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_