No, it isn't the body of an anonymous function. It is an initialization list.. and it serves to set the fields of the new object of type GoogleAuthorizationCodeFlow.Initializer
all in-line.
It is the "in-line" version of this:
var initializer = new GoogleAuthorizationCodeFlow.Initializer();
initializer.DataStore = new FileDataStore("Tasks.ASP.NET.Sample.Store");
initializer.ClientSecretsStream = stream;
initializer.Scopes = new[] { TasksService.Scope.TasksReadonly };
flow = new GoogleAuthorizationCodeFlow(initializer);
The two are equivalent functionally. It is just more compact.