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

[Kim Barrett]

> On Feb 16, 2018, at 8:40 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> 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

The change to add the optional arguments looks good.  However, why do
we have GCTimer::register_gc_pause_start/end?  Couldn't
ConcurrentGCTimer::register_gc_pause_start/end just call (public)
register_gc_phase_start/end directly?