Make all ExternalStrings bail out JIT code when accessing the sting contents

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 9104d60a099574cf768ac86fe7ad2b35e21aa0f5..664bcf828b36700dde7d3fb8a76b5396c1ea0f98 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -647,7 +647,7 @@ MaybeHandle<String> Factory::NewExternalStringFromOneByte(
     THROW_NEW_ERROR(isolate(), NewInvalidStringLengthError(), String);
   }
 
-  Handle<Map> map = external_one_byte_string_map();
+  Handle<Map> map = short_external_one_byte_string_map();
   Handle<ExternalOneByteString> external_string =
       New<ExternalOneByteString>(map, NEW_SPACE);
   external_string->set_length(static_cast<int>(length));
@@ -671,7 +671,8 @@ MaybeHandle<String> Factory::NewExternalStringFromTwoByte(
   bool is_one_byte = length <= kOneByteCheckLengthLimit &&
       String::IsOneByte(resource->data(), static_cast<int>(length));
   Handle<Map> map = is_one_byte ?
-      external_string_with_one_byte_data_map() : external_string_map();
+      short_external_string_with_one_byte_data_map()
+      : short_external_string_map();
   Handle<ExternalTwoByteString> external_string =
       New<ExternalTwoByteString>(map, NEW_SPACE);
   external_string->set_length(static_cast<int>(length));