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

components/sync/model/model_error.h

Index: components/sync/model/model_error.h
diff --git a/components/sync/model/model_error.h b/components/sync/model/model_error.h
index ebd0dae57076d0b5e2c5fb5b5fd90204edd6e3d9..6c3536e4ca41add916bdd09dfe75f216f17a750d 100644
--- a/components/sync/model/model_error.h
+++ b/components/sync/model/model_error.h
@@ -15,18 +15,12 @@ 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;
@@ -36,7 +30,6 @@ class ModelError {
   const std::string& message() const;
 
  private:
-  bool is_set_;
   tracked_objects::Location location_;
   std::string message_;
 };