n-api: directly create Local from Persistent

The `v8::PersistentBase<T>.Get` method didn't exist in node 4 and it's
just a helper which creates a new `v8::Local` from the given object.

PR-URL: https://github.com/nodejs/node/pull/14211
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>

src/node_api.cc

@@ -56,7 +56,8 @@ struct napi_env__ {
       (destination)->SetInternalFieldCount((field_count));         \
       (env)->prefix ## _template.Reset(isolate, (destination));    \
     } else {                                                       \
-      (destination) = env->prefix ## _template.Get(isolate);       \
+      (destination) = v8::Local<v8::ObjectTemplate>::New(          \
+          isolate, env->prefix ## _template);                      \
     }                                                              \
   } while (0)