RFR: 8209802: Garbage collectors should register JFR types themselves to avoid build errors.

[Stefan Johansson]

Hi,

Please review this patch to fix JDK-8209802.

JBS: https://bugs.openjdk.java.net/browse/JDK-8209802
Webrev: http://cr.openjdk.java.net/~sjohanss/8209802/00/

Summary:
This patch builds on the cleanup made in JDK-JDK-8230431, and adds an initialize method to G1NewTracer, that takes care of registering serializers needed for the two G1-specific constants in JFR. There is also a G1 periodic event but there is no similar way to register this from G1 code so it needs to be solved by guarding the G1 specific code in jfrPeriodic.cpp. 

Testing:
Mach5 tier 1-3

Thanks,
Stefan