RFR (XS) 8006851: When TieredCompilation is set, max code cache should be bumped to 256mb

[Morris Meyer]

Folks,

I'd like to get this small change reviewed.  Per the workflow this has 
gone through JPRT.

Thanks in advance,

         --morris

JBS - https://jbs.oracle.com/bugs/browse/JDK-8006851
WEBREV - http://cr.openjdk.java.net/~morris/8006851