To follow up on what Hao Kung suggested.
You will find the line of code in question in the accountController.cs file. The default function is:
private async Task SignInAsync(ApplicationUser user, bool isPersistent)
{
AuthenticationManager.SignOut(DefaultAuthenticationTypes.ExternalCookie);
var identity = await UserManager.CreateIdentityAsync(user, DefaultAuthenticationTypes.ApplicationCookie);
AuthenticationManager.SignIn(new AuthenticationProperties() { IsPersistent = isPersistent}, identity);
}
To enable the remember me functionality with all external providers, change the line:
AuthenticationManager.SignIn(new AuthenticationProperties() { IsPersistent = isPersistent}, identity);
by changing the isPersistent varible to the constant true:
AuthenticationManager.SignIn(new AuthenticationProperties() { IsPersistent = <b>true</b>}, identity);