RFR: [8] Missing event log for canceled GC

[Aleksey Shipilev]

This is sh/jdk8u specific: there seems to be a missing event block for canceled GC. This was the
mistake during the backports.

$ hg qdiff
diff -r 590832e3234c src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp
--- a/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp	Mon Feb 26 18:00:18 2018 +0100
+++ b/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp	Wed Feb 28 11:01:27 2018 +0100
@@ -1704,7 +1704,9 @@

 void ShenandoahHeap::cancel_concgc(GCCause::Cause cause) {
   if (try_cancel_concgc()) {
-    log_info(gc)("Cancelling concurrent GC: %s", GCCause::to_string(cause));
+    FormatBuffer<> msg("Cancelling concurrent GC: %s", GCCause::to_string(cause));
+    log_info(gc)("%s", msg.buffer());
+    Events::log(Thread::current(), "%s", msg.buffer());
     _shenandoah_policy->report_concgc_cancelled();
   }
 }

Testing: hotspot_gc_shenandoah, eyeballing hs_errs

Thanks,
-Aleksey