Allow externalizing cons and sliced string.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 7098859de3cfa9fa85d786b29cf8eef56846f086..8dbe0a1c10c2c20f6dda26a895f13c8faa04227f 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5541,11 +5541,6 @@ 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;