🏄 [heap] Add page evacuation mode for new->old

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 4d27c40f06ea7f7e1be2c112e89a5635a9db3c3f..2942b018345c01aa170b98323c35df342714c174 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1401,6 +1401,8 @@ class Heap {
   // finalize them.
   class ExternalStringTable {
    public:
+    enum CleanupMode { kFull, kPromoteOnly };
+
     // Registers an external string.
     inline void AddString(String* string);
 
@@ -1408,6 +1410,7 @@ class Heap {
 
     // Restores internal invariant and gets rid of collected strings.
     // Must be called after each Iterate() that modified the strings.
+    template <CleanupMode mode = kFull>
     void CleanUp();
 
     // Destroys all allocated memory.