Revert 23058 - "Allow externalizing cons and sliced string."

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 6f6fb42a32484b588e986429f2e3b47b125be8b4..aaf6865bc8e6044b551f4e3bf81751494b3159e7 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5551,6 +5551,11 @@ bool v8::String::CanMakeExternal() {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
 
+  // TODO(yangguo): Externalizing sliced/cons strings allocates.
+  // This rule can be removed when all code that can
+  // trigger an access check is handlified and therefore GC safe.
+  if (isolate->heap()->old_pointer_space()->Contains(*obj)) return false;
+
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) return false;
   int size = obj->Size();  // Byte size of the original string.
   if (size < i::ExternalString::kShortSize) return false;