Introduce --zap-cpp-pointers (off by default)

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index f7acf9c0079bec587c73f9b9c9cb96c37878d1c0..61d467f219b47aebe08addf98a2ca07f03b10c4e 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8723,6 +8723,7 @@ class Relocatable BASE_EMBEDDED {
   static char* Iterate(RootVisitor* v, char* t);
 
  private:
+  friend class HeapPointerZapper;  // For |prev_|.
   Isolate* isolate_;
   Relocatable* prev_;
 };
@@ -8735,7 +8736,7 @@ class FlatStringReader : public Relocatable {
  public:
   FlatStringReader(Isolate* isolate, Handle<String> str);
   FlatStringReader(Isolate* isolate, Vector<const char> input);
-  void PostGarbageCollection();
+  void PostGarbageCollection() override;
   inline uc32 Get(int index);
   template <typename Char>
   inline Char Get(int index);