RFR 8176350: Usage constraints don't take effect when using PKIX

[Anthony Scarpino]

Hi,

I need a code review of this small change.. The PKIX path for usage 
checking didn't pass the variant to the checkers because of a previous 
needed check for plugins.

http://cr.openjdk.java.net/~ascarpino/8176350/webrev/

thanks

Tony