Don't overwrite existing serial numbers on the function template, otherwise...

Don't overwrite existing serial numbers on the function template, otherwise instantiating the function for a new context causes the serial number to bump.

Review URL: https://codereview.chromium.org/988693003

Cr-Commit-Position: refs/heads/master@{#27048}

src/api.cc

@@ -943,7 +943,7 @@ static Local<FunctionTemplate> FunctionTemplateNew(
   InitializeFunctionTemplate(obj);
   obj->set_do_not_cache(do_not_cache);
   int next_serial_number = 0;
-  if (!do_not_cache) {
+  if (!do_not_cache && !obj->serial_number()->IsSmi()) {
     next_serial_number = isolate->next_serial_number() + 1;
     isolate->set_next_serial_number(next_serial_number);
   }