Just calculate pretenuring ratio when needed.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index a0256fb416429940c41002429ae7731c6024e984..dbac0f4685e2d0e594014e97410698499d5b84ff 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1551,10 +1551,13 @@ inline bool AllocationSite::DigestPretenuringFeedback() {
   bool decision_changed = false;
   int create_count = memento_create_count();
   int found_count = memento_found_count();
-  double ratio = static_cast<double>(found_count) / create_count;
+  bool minimum_mementos_created = create_count >= kPretenureMinimumCreated;
+  double ratio =
+      minimum_mementos_created || FLAG_trace_pretenuring_statistics ?
+          static_cast<double>(found_count) / create_count : 0.0;
   PretenureFlag current_mode = GetPretenureMode();
 
-  if (create_count >= kPretenureMinimumCreated) {
+  if (minimum_mementos_created) {
     PretenureDecision result = ratio >= kPretenureRatio
         ? kTenure
         : kDontTenure;