src: make env_ and context_ private

This commit makes the currently protected members env_ and context_
private in node_contextify.h.

PR-URL: #20671
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_contextify.h

@@ -15,10 +15,6 @@ struct ContextOptions {
 };
 
 class ContextifyContext {
- protected:
-  Environment* const env_;
-  Persistent<v8::Context> context_;
-
  public:
   ContextifyContext(Environment* env,
                     v8::Local<v8::Object> sandbox_obj,
@@ -97,6 +93,8 @@ class ContextifyContext {
   static void IndexedPropertyDeleterCallback(
       uint32_t index,
       const v8::PropertyCallbackInfo<v8::Boolean>& args);
+  Environment* const env_;
+  Persistent<v8::Context> context_;
 };
 
 }  // namespace contextify