RFR (S): 8234208: Logging reports zero total refined cards under "Before GC RS summary"

[Man Cao]

Hi all,

Can I have reviews for this fix for GC logging in G1RemSetSummary?
Webrev: https://cr.openjdk.java.net/~manc/8234208/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8234208

This fix makes G1RemSetSummary independent of G1Policy, which in my opinion
is cleaner than reordering the
callsites of policy()->record_collection_pause_start() and
rem_set()->print_periodic_summary_info(). The cost of summing up the total
number of refined cards should be negligible.

I also removed a null check for _rs_threads_vtimes, which is unnecessary
after JDK-8183226.

-Man