async_hooks: make sure .{en|dis}able() === this

Make sure that `hook.enable()` and `hook.disable()` return `hook`
consistently, as the documentation indicates.

PR-URL: https://github.com/nodejs/node/pull/13418
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/async_hooks.js

@@ -96,7 +96,7 @@ class AsyncHook {
 
     // Each hook is only allowed to be added once.
     if (hooks_array.includes(this))
-      return;
+      return this;
 
     if (!setupHooksCalled) {
       setupHooksCalled = true;
@@ -124,7 +124,7 @@ class AsyncHook {
 
     const index = hooks_array.indexOf(this);
     if (index === -1)
-      return;
+      return this;
 
     hook_fields[kInit] -= +!!this[init_symbol];
     hook_fields[kBefore] -= +!!this[before_symbol];

test/parallel/test-async-hooks-enable-disable.js

@@ -0,0 +1,20 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const async_hooks = require('async_hooks');
+
+const hook = async_hooks.createHook({
+  init: common.mustCall(() => {}, 1),
+  before: common.mustNotCall(),
+  after: common.mustNotCall(),
+  destroy: common.mustNotCall()
+});
+
+assert.strictEqual(hook.enable(), hook);
+assert.strictEqual(hook.enable(), hook);
+
+setImmediate(common.mustCall());
+
+assert.strictEqual(hook.disable(), hook);
+assert.strictEqual(hook.disable(), hook);
+assert.strictEqual(hook.disable(), hook);