RFR(XXS): 7147806: G1: Crash in vm bootstrap when running with -XX:+UseG1GC -XX:-UsePerfData

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers to review the changes for this CR? The 
webrev can be found at: http://cr.openjdk.java.net/~johnc/7147806/webrev.0/

The issues here was that some updates to the G1 performance counter 
variables were not guarded. In the other collectors, the generation and 
collector performance counters are allocated/initialized 
unconditionally. The performance counter variables that these counters 
contain, however, are allocated and updated only if UsePerfData is 
enabled. In G1, some of these updates were not suitably guarded and so 
we tried to update some unallocated performance counter variables.

Testing:
* Small test case (gcbasher) with the offending command line flag 
combination and with just G1.

Thanks,

JohnC