Add trace output when recompilation queues are flushed.

src/optimizing-compiler-thread.cc

Index: src/optimizing-compiler-thread.cc
diff --git a/src/optimizing-compiler-thread.cc b/src/optimizing-compiler-thread.cc
index 1f72cc074e6d2de70500423464006063cf6cdb26..e6467f1547f641c60c8de772726760eaa3d8bc53 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -172,6 +172,9 @@ void OptimizingCompilerThread::Flush() {
   stop_semaphore_.Wait();
   FlushOutputQueue(true);
   if (FLAG_concurrent_osr) FlushOsrBuffer(true);
+  if (FLAG_trace_concurrent_recompilation) {
+    PrintF("  ** Flushed concurrent recompilation queues.\n");
+  }
 }