Uncommit and shrink semi-spaces only on low allocation rate.

src/heap/gc-idle-time-handler.h

Index: src/heap/gc-idle-time-handler.h
diff --git a/src/heap/gc-idle-time-handler.h b/src/heap/gc-idle-time-handler.h
index 5c0a80a1d25653107afac42b393bc16179977646..e5b671cd063d9b4e36444a23fc8908dfbda07ab9 100644
--- a/src/heap/gc-idle-time-handler.h
+++ b/src/heap/gc-idle-time-handler.h
@@ -188,6 +188,7 @@ class GCIdleTimeHandler {
     size_t used_new_space_size;
     size_t new_space_capacity;
     size_t new_space_allocation_throughput_in_bytes_per_ms;
+    size_t current_new_space_allocation_throughput_in_bytes_per_ms;
   };
 
   GCIdleTimeHandler()