Review request JMC-5701: GC stall rule never reports OK result

[Florian David]

Hi !

Please find a review to fix the GC stall rule.

JIRA: https://bugs.openjdk.java.net/browse/JMC-5701
Webrev: http://cr.openjdk.java.net/~hirt/fdavid/JMC-5701/webrev.0/

Regards,
Florian