RFR: 8205344: TraceMemoryManagerStats changes in JDK-8195115 broke ZGC

[Per Liden]

JDK-8195115 changed the TraceMemoryManagerStats constructor interface 
but failed to take ZGC into account, causing failure in Tier2 testing. 
This patch should restore the previous behavior for ZGC.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205344
Webrev: http://cr.openjdk.java.net/~pliden/8205344/webrev.0

Testing: With this patch I can no longer reproduce locally. Running in 
mach5 t{1,2,3} right now.

/Per