Add more logging for debugging possible deadlock

Change-Id: I4b2db54504d3ed8c2c33a110191488cab1fcb2d9
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 56919bd..f8df222 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -83,6 +83,9 @@
       LOG(INFO) << "Verifying no from-space refs";
     }
     VerifyNoFromSpaceReferences();
+    if (kVerboseMode) {
+      LOG(INFO) << "Done verifying no from-space refs";
+    }
     CheckEmptyMarkQueue();
   }
   {