Thank you for your response Christian.
The requirement of directly closing the browser has come as a result of not being able to clear SSO cookies after logoff. For logoff, we're calling the standard SAP logoff ICF service, ie, on the server side. Therefore, only after closing the browser can we invalidate the SSO cookies. This problem is only specific to newer versions of chrome [we have a workaround for IE or Firefox].
Please let me know if you know a workaround to clear SSO cookies specific for current chrome browser versions.
Thank you.