RFR(s): 8198268: Add time argument to ConcurrentGCTimer::register_gc_pause_start/_end

[Per Liden]

Patch to allow a GC to register a time when calling 
ConcurrentGCTimer::register_gc_pause_start/end. This makes the 
ConcurrentGCTimer API more in line with all other register_* functions 
in GCTimer and ConcurrentGCTimer, which all take an optional time argument.

Bug: https://bugs.openjdk.java.net/browse/JDK-8198268
Webrev: http://cr.openjdk.java.net/~pliden/8198268/webrev.0/

/Per