🏄 [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 59d7dfcf002f9229018a4965cdc0617eeef70541..565f236c6cca81e5cf39b611d3dcd2da705aeb5b 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1408,6 +1408,8 @@ class Heap {
   // finalize them.
   class ExternalStringTable {
    public:
+    enum CleanupMode { kFull, kPromoteOnly };
+
     // Registers an external string.
     inline void AddString(String* string);
 
@@ -1415,6 +1417,7 @@ 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.