Implement high promotion mode for new space.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 9f4f505e1d9d04a6695b1cd2d92a776e982263d0..081c4663de49df6b48ab7c3a251c29c24409d0d0 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1569,6 +1569,10 @@ class Heap {
   HeapDebugUtils* debug_utils_;
 #endif  // DEBUG
 
+  // Indicates that the new space should be kept small due to high promotion
+  // rates caused by the mutator allocating a lot of long-lived objects.
+  bool new_space_high_promotion_mode_active_;
+
   // Limit that triggers a global GC on the next (normally caused) GC.  This
   // is checked when we have already decided to do a GC to help determine
   // which collector to invoke.
@@ -1809,6 +1813,10 @@ class Heap {
     return survival_rate_trend() == INCREASING;
   }
 
+  bool IsDecreasingSurvivalTrend() {
+    return survival_rate_trend() == DECREASING;
+  }
+
   bool IsHighSurvivalRate() {
     return high_survival_rate_period_length_ > 0;
   }