[builtins] migrate C++ String Iterator builtins to baseline TurboFan

src/objects-debug.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/objects.h"
 
 #include "src/bootstrapper.h"
 #include "src/disasm.h"
 #include "src/disassembler.h"
 #include "src/field-type.h"
@@ skipping 767 matching lines @@
 void JSWeakMap::JSWeakMapVerify() {
   CHECK(IsJSWeakMap());
   JSObjectVerify();
   VerifyHeapPointer(table());
   CHECK(table()->IsHashTable() || table()->IsUndefined(GetIsolate()));
 }
 
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
-  CHECK(string()->IsString());
+  CHECK(string()->IsSeqString() || string()->IsExternalString());
+
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);
 }
 
 void JSWeakSet::JSWeakSetVerify() {
   CHECK(IsJSWeakSet());
   JSObjectVerify();
   VerifyHeapPointer(table());
   CHECK(table()->IsHashTable() || table()->IsUndefined(GetIsolate()));
 }
@@ skipping 564 matching lines @@
 
   // Both are done at the same time.
   CHECK_EQ(new_it.done(), old_it.done());
 }
 
 
 #endif  // DEBUG
 
 }  // namespace internal
 }  // namespace v8