Improve generalization / migration tracing.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 169475791d05ff9672dfff163ba14dde0506bea5..6c1ecbb322ea00211813ba6c9584fcbf8a227b5b 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1567,14 +1567,27 @@ MaybeObject* JSObject::MigrateInstance() {
   // Converting any field to the most specific type will cause the
   // GeneralizeFieldRepresentation algorithm to create the most general existing
   // transition that matches the object. This achieves what is needed.
-  return GeneralizeFieldRepresentation(0, Representation::None());
+  Map* original_map = map();
+  MaybeObject* maybe_result = GeneralizeFieldRepresentation(
+      0, Representation::None());
+  JSObject* result;
+  if (FLAG_trace_migration && maybe_result->To(&result)) {
+    PrintInstanceMigration(stdout, original_map, result->map());
+  }
+  return maybe_result;
 }
 
 
 MaybeObject* JSObject::TryMigrateInstance() {
   Map* new_map = map()->CurrentMapForDeprecated();
   if (new_map == NULL) return Smi::FromInt(0);
-  return MigrateToMap(new_map);
+  Map* original_map = map();
+  MaybeObject* maybe_result = MigrateToMap(new_map);
+  JSObject* result;
+  if (FLAG_trace_migration && maybe_result->To(&result)) {
+    PrintInstanceMigration(stdout, original_map, result->map());
+  }
+  return maybe_result;
 }