RFR: 8229135: ZGC: Adding missing ZStatTimerDisable before call to ZVerify::roots_strong()

[Per Liden]

ZVerify::roots_strong() is called outside of a ZStatTimerDisable scope, 
which means the root scanning stat counters/samplers will be polluted.

(This fix was originally part of JDK-8229017, but Stefan asked me to 
break this out into a separate fix)

Bug: https://bugs.openjdk.java.net/browse/JDK-8229135
Webrev: http://cr.openjdk.java.net/~pliden/8229135/webrev.0

/Per