I've had the same problem the last few days. Prior to these last few days, I had the option to proceed anyway, but not anymore. There is only the back button. I even went in using Firefox and went to my Twitter settings and removed the check on Always Use HTTPS. Under it is says "Use a secure connection where possible to encrypt your account information.". I then saved my settings and came back to Chrome to try to go to Twitter and no luck. There is no option on the Security Error screen that allows me the option to proceed anyway as before. This is ridiculous. I don't want to switch back to FF for Twitter only, but if this is not fixed I will go back to using FF for EVERYTHING!
I have also cleared all of Twitter's cookies, added both twitter.com and https://twitter.com to my exceptions list, cleared my cache. When you click on the details of the certificate, and then click on Certification Path, it says at the bottom under Certification Status, that "This certificate is ok."